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Your midterm exam consists of 6 essay questions worth 15 points, covering chapters 1-11 of your textbook. Which 6 questions are your choice; you may pick any 6 questions you like, but you may not answer more than 6.

Answering less than 6 or more than 6 will result in points lost.

Each question you choose to answer must be answered thoroughly,

with referenced research to back up your answer

. Questions are primarily from the textbook. Use proper APA citing (in-text) and references. A combination of the text and academic sources should be used to complete your essay answers. Non-academic sources like wikipedia.org will not be accepted. The answer must be as complete as possible,

minimum of 250 words in essay format

. Overly brief answers will result in points lost. Papers will be held to the strictest professional standards, so watch your spelling and grammar; proofread your work!

Use APA formatting for the text body and citations/works cited.

See Rubrics in Canvas for the allocation of points.

Submit your answers as one Word document, numbering your answers.

Timeliness is a learning outcome of this course. You will lose 10 points per day for lateness up to 50% of the grade.

Choose 6 out of the 10 questions from the following:

Chapter 1:

Discuss the reasons for the downturn in the number of pilots from the late 1970s through the early 1990s. Describe the challenges faced by the industry in light of the decline in all segments of the general aviation community. How did the government agencies, manufacturers, and the general aviation community respond to these challenges?

Chapter 2:

Describe several services provided by the FAA for general aviation pilots. What is the purpose of flight service stations? What has been the trend in general aviation aircraft manufacturing since 1980? Identify some single- and multi-engine aircraft available in the United States. What is the significance of pilots to aircraft manufacturing? Identify and describe several sources for general aviation forecasts.

Chapter 3:

In establishing an FBO, what are the advantages and disadvantages of the three main forms of legal structure? Describe the importance of analyzing the market and selecting a location. What is market analysis? Discuss the importance of the community. Describe the factors which must be considered in selecting a site for an FBO. Where can a prospective FBO get assistance?

Chapter 4:

Why is fire safety important for the line service specialist? What are the four classes of fires? Describe in detail. What principles are used in extinguishing a fire? What measures should be taken by FBOs to minimize the risk of fire?

Chapter 5-6:

You have been anxiously awaiting this day – the grand opening of your FBO. It’s not going to be easy, though, because there is an established FBO already on the field. What can you do to differentiate your FBO from the competition? Be specific, factoring in what you learned from the course so far.

Chapter 7:

As a Marketing Manager for an up-start FBO, you have been tasked with establishing a marketing mix for flight instruction. Considering the “4 Ps”, how would you do that

new assignment here

Discussion Questions:

1. (CH 10) The president of the organization in the first question is seriously considering the purchase of the business aircraft that you recommended, but is still undecided. From the attached links found in the Reading Assignment Page of this module, explain the utilization strategies, benefits, and value of owning a business aircraft.2. (CH 11) Define fractional ownership and explain the advantages and disadvantages. Find a fractional ownership company and discuss the programs, aircraft, and services they offer to their owners.

New assignment here

Research one airplane accident (general aviation airplane and one air transportation size airplane) involving the components described in Chapter 10.  Compare and contrast your landing gear systems from the text to the accident report or news articles in

a minimum of 1 page posted on the discussion page

. Use proper APA citing (in-text) and references. A combination of the text, NTSB accident reports or summaries, aviation publications articles, and the PHAK should be used to complete your case study.

Non-academic sources like wikipedia.org will not be accepted

. An overly brief case study will result in points lost.Papers will be held to the strictest professional standards, so watch your spelling and grammar; proofread your work! Use APA formatting for the text body and citations/works cited. 75% of your discussion board grade will be based on formatting and quality of content and 25% will be based on participation

(timely posting and two replies to fellow students by Sunday).

New Assignment here

Papers will be held to the strictest professional standards, so watch your spelling and grammar; proofread your work! Use APA formatting for the text body and citations/works cited. See Rubrics in Canvas for the allocation of points.Include the question with your answer in your Word document.

The completed assignment must be uploaded as a Word Document onto Canvas.   If your work is submitted late work 10 points per day will be deducted.

Explain the differences between the series circuits, parallel circuits, and series-parallel circuits.

Describe the terms Volt, Amps, and Amp-Hour. How do they apply to the theory of battery operation?

Discuss how the ammeter is a helpful diagnostic tool to a pilot.

Why is the phrase “ Electrical-System Failure” commonly misused? Explain what occurs during an alternator failure and what is the trick is to identify the problem?

Describe how the pilot would troubleshoot an alternator failure. What steps should a pilot take when making load-shedding decisions.

Read the Flying article by J. Mac McClellan (May 7, 2010) and answer the questions after the article link below.

https://www.flyingmag.com/safety/maintenance/understanding-electrical-systems (Links to an external site.)

Describe a few electrical advances in newer airplanes.  Explain the NTSB’s concern about resetting circuit breakers.  Identify and describe

11
Environmental Systems
L
ARGE TURBINE-ENGINE AIRCRAFT, WITH THEIR SOPHISTICATED
environmental systems, are able to isolate occupants from outside weather. Un­
fortunately, the light airplane traveler does not fare quite so well.
For one thing, the airlines literally have a trick up their sleeve: It’s called an airport jet-
way. Even the most sophisticated air conditioning and heating systems can’t keep a cabin
comfortable if it has a door open to the world. The jetway cleverly forms a sleeve that con­
nects the airplane to the terminal, allowing maintenance of cabin temperature.
At terminals without jetways, a quick turnaround reduces the time cabin doors are
open. Then, too, airline cabins are long and have several bulkhead, minimizing the flow of
air out the door. The relatively small size of the average general aviation fuselage makes
the inside temperature sensitive to open cockpit doors. And the inadequacy of heating and
cooling systems precludes the luxury of a stable cabin environment unless you are able to
load and unload passengers in a temperature-controlled hangar.
TYPES OF AIRCRAFT HEATERS
Aircraft powered by turbine engines have a ready source of heat, the turbine engine
itself. Reciprocating engine-powered aircraft do not have that available so cabin heat­
ing must come from another source. There are two types of heaters for light, generalaviation airplanes: an exhaust-manifold heater and a combustion heater.
Chapter Eleven
Exhaust Manifold Heater
The exhaust manifold heater, used exclusively be reciprocating, single-engine aircraft,
is the simplest. Working on a simple heat-transfer principle, a shroud is placed around
the engine exhaust stack as illustrated in Figure 11-1. Fresh outside air is forced by ram
pressure through the shroud and around the exhaust stack. The stack isolates the exhaust
gas from the fresh air but allows the heat to transfer. To vary the temperature the pilot
controls a source of additional outside air that mixes with the heated air to cool it to the
desired temperature.
The main advantages of the exhaust manifold heater are simplicity, low mainte­
nance, and virtually no reduction in flight performance. It doesn’t consume fuel, reduce
engine power, or decrease airspeed by any detectable amount. The disadvantages, how­
ever, are obvious to anyone who has ever flown in a lightplane during winter. An exhaust
manifold heater is a very ineffective system on the ground because minimal ram air
moves through the shroud. Not only does this mean the cabin tends to stay cold during
ground operations, it also means poor windshield defrosting. Because windshield de­
frosting is accomplished by the rechanneling of cabin heat, it is common for pilots to taxi
and take off with fogged windshields, peering through small circles smeared away with
the back of their hands, leaving nose prints on the acrylic.
Fresh Air
o
Fig. 11-1. Exhaust manifold heater.
240
Cabin Heat
Environmental Systems
There is also a threat of carbon monoxide (CO) poisoning if fc
exhaust stack leaks
,ns.de the shroud. Dunng preflight, .t is always a good idea to check exhaust-stack seam
to assure that the welds are solid; any leakage within the engine compartment may cause
the cabin to fill with CO. It is impossible to check the exhaust weld within the heater
shroud, though, and it provides a direct route for CO to travel into the cabin The best
safety measure is to put an inexpensive carbon monoxide detector in every airplane
When exposed to CO, the colored pad turns from orange to black in fifteen minutes or
less, depending on the amount of carbon monoxide, alerting the pilot to a serious hazard.
Combustion Heater
If you think there ought to be a better way, you’re right; there is. It’s called a combustion
heater, and most multiengine airplanes have one. But why don’t singles? For the most
part, airframe manufacturers feel it is too expensive to put combustion heaters in singleengine aircraft. So, at least for the time being, only multiengine pilots will have the lux­
ury of instant heat.
Piper installs Janitrol combustion heaters exclusively in all of its twin-engine aircraft
built after 1964. Janitrol shares the rest of the market with Stewart Warner’s Southwind
heater. The fundamentals of aircraft combustion heating haven’t changed much over the
years. According to the Aircraft Heating Digest, Volume 1, Number 1, published by Jan­
itrol Aircraft in February, 1949, there are four main requirements: fuel for combustion,
air for combustion, ignition to start combustion, and air to carry away the heat produced
by combustion. That was true for the DC-3, and it’s still true for modem aircraft.
Heat is produced by burning a fuel/air mixture in a heater combustion chamber. This
is somewhat of a mixed blessing, because while it conveniently uses fuel drawn from the
aircraft fuel tanks, it also reduces the aircraft’s range when the heater is in use. Nonethe­
less, when you get into a cabin that is below freezing and can have near instantaneous heat
without even starting an engine, a little less range—at least to this northerner—doesn’t
seem so bad.
The Janitrol heater uses a spray nozzle to send regulated, atomized fuel/air mixture
into the heater combustion chamber. There, a high-voltage spark plug powered by the air­
craft’s electrical system provides continuous ignition. Because aircraft attitude and alti­
tude always are subject to change—sometimes rapidly—Janitrol uses what it calls the
“whirling flame” principle. The fuel/air mixture enters the combustion chamber tangent
to the chamber’s surface as depicted in Figure 11-2. This forces the airflow to spin and
mix with itself, causing a stable, continuous flame pattern. The burning gases flow the
length of the combustion tube, double back over the outside of the chamber, go through
a crossover passage to an outer radiating area, travel down the length ot the heater one
more time, and finally exit through the exhaust.
,
,
The cabin ventilation is ducted separately between the combustion air chambers.
Though the two airflows never mix (to do so would lead to CO poisoning), t e ven i a
ing air does contact several surfaces heated by the combustion air, causing eat trans; e .
Several other components round out the system. An electric fuel pump is nece
241
Chapter Eleven
Flame
Combustion Air
Inlet
Fuel Inlet
Healed Air
Solenoid
Valve
Exhaust Gases
Fresh Air
from Blower
Fig. 11-2. Janitrol heater cutaway.
though some aircraft actually may use the engine-driven fuel pump if the fuel pressure
output is correct. There must be a ventilating air blower, which also doubles as a cooling
fan when the heater is turned off. and a separate combustion air blower. Temperature com
tr°I is maintained by either a duct switch or cycling switch, which senses heat output and
compares it to the selected temperature. And, of course, there are the requisite controls
and lights that indicate the operational status of the heater.
Preflight
System preflight should include checking for either blockage or damage to both the ven­
tilation and combustion air inlets, the heater-exhaust outlet, and the heater fuel drain. Ad­
ditionally, the area around the heater exhaust tube should be checked routinely for soot
accumulation; this indicates an excessively rich fuel/air mixture (which is caused by in­
correct fuel pressure), a blocked combustion-air inlet line, an inoperative or failed com­
bustion blower, or a clogged fuel nozzle.
In addition to the visual preflight, an operational check should be accomplished,
irst, turn the heater master switch on and assure that the ventilation and combustion
blowers work; the heater-failure light also should illuminate, indicating that the system is
activated, but there is no combustion. With the master switch still on, check for excessive
a”y “nusual
ZT’H
or noises. Then perform the operational check as
outlined in your pilot s operating handbook (POH).
Troubleshooting
HeateSil^nS3′
run- heater fires’h
to s’hut off
242
t
C.m
°CCUr that retluire troubleshooting on your part:
.VentI,at’ng air b,ower fails
1
to run; combustion air blower fails to
Steadily; heater starts’ then S°es out; and
heater fails
Environmental Systems
If the heater fails to light, the first consideration is procedure; double-check the POH
to make sure you are using the correct one. Beyond that, insufficient electrical power
such as a dead battery or insufficient fuel, should be suspected. Some mechanical prob­
lems are beyond the control of the pilot, such as restricted fuel nozzle or inoperative fuel
pump.
Failure of the ventilating blower to run probably will mean you forgot to turn the
heater master switch on. Otherwise, it’s a mechanic’s job, as is failure of the fully auto­
matic combustion air blower.
When the heater fires up but doesn’t bum steadily, the culprit probably is fuel
related—an insufficient amount or contamination by ice or water. Other mechanical
problems, such as a fouled spark plug, can produce the same results.
If you suddenly realize that you can see your breath and you haven’t had garlic for
lunch, the heater has probably gone out. While mechanical problems could be the cause,
more likely the problem is either fuel or electrical starvation. Best bet is to check the fuel
supply and master switch.
Finally, if the heater fails to shut off during shutdown, it is a mechanic’s problem
such as a defective heater switch or stuck fuel solenoid valve.
The crux of the matter is proper maintenance. Preflight, preventive, and periodic main­
tenance are the keys to efficient and safe operation. Your zero-time overhauled Janitrol
heater is certified to run 500 hours (or 24 months, whichever comes first) in accordance
with Janitrol AD Note #96-20-07 before a pressure decay test is required. After that, every
100 hours (or 24 months, whichever comes first) another decay test is due. To avoid having
to continually comply with this airworthiness directive, you can replace the unit with a new
JanAero extended-life heater assembly, which consists of a ceramic coated combustion
tube. Performed by a mechanic, the preventive maintenance is a thorough inspection of the
entire unit, including a pressure check of the combustion chamber.
Southwind also has a maintenance-related airworthiness directive. Southwind AD
#81-09-09 requires a 250-hour inspection and, at 1000 hours time in service, it must be
overhauled in accordance with the manual. Incidentally, it is important to note that the
“hours” referred to are actual heater-operation hours. While some aircraft have a heater
hour-meter that records operating time, many do not. Janitrol allows the operator to com­
pute one hour of heater operating time as the equivalent of two flight hours. Southwind,
on the other hand, requires straight flight hour time if you don’t have an hour meter on
the unit itself. In general, it is probably going to be cost-effective to have an hour meter
installed on all heaters.
R CONDITIONING
wing up in the 1950s, a frequent sight was “It’s Cool Inside” emblazoned across themarquees. The lure of air-conditioning, something unheard of in homes, was e n o g
ttract crowds on any hot summer night regardless of the picture that was s
ditioning spread to restaurants, to other public places, and finally to
omes
_
•ed to heat, consumers quickly began to expect to be kept coo in oors- P
nd their way into cars and finally commercial aircraft. If asked, the average pilot would
Chapter Eleven
probably tell you it isn’t practical to air-condition small aircraft. We tend to be most con­
cerned about protecting the pilot from extreme cold, forgetting that excessive heat is also
a problem.
Studies conducted by both the U.S. Air Force and the U.S. Army show that an air­
plane with a stable, comfortable cabin temperature is a safer flying environment. With
30-minute waits on the ground at some of the larger airports, and outside air temperatures
of 80-90 degrees Fahrenheit (F), it’s no wonder that the inside of an airplane can exceed
100 degrees. How safe can a pilot be after sitting in a 100-degree cabin for 30 minutes
prior to takeoff? Certainly, few business executives are going to sit in that kind of heat.
There are penalties to be paid for air-conditioning, to be sure. There is an increase in
aircraft empty weight, due to the compressor and other required equipment. This trans­
lates into fewer bags, reduced fuel, or fewer passengers. Just the operation of the system
causes a reduction in available engine horsepower. The Cessna 2ION operating hand­
book states that there is a one-knot TAS cruise reduction when air-conditioning is in­
stalled on the aircraft, and an additional one to two-knot TAS cruise reduction when the
compressor actually is operating!
Two types of heat affect airplanes: aerodynamic and sun. Aerodynamic, also known
as adiabatic skin temperature, is the result of free-stream kinetic energy being converted
to thermal energy when the free stream air is slowed to zero at the surface of the airplane.
The faster the airplane moves through the air, the greater the heat buildup, skin tempera­
ture being a function of free-stream temperature and Mach number. For instance, at
Mach 2.0, the fuselage temperature would be approximately 260 degrees F; at Mach 5.0,
it would be about 1550. This obviously is a problem for large aircraft, not singles or light
twins. For the slower aircraft, the basic problem is the sun and little or no ventilation to
carry off cabin heat. The automotive air-conditioner fits nicely into this type of airplane.
System Overview
Fundamentally, air-conditioning is simple physics; the rapid expansion of fluid causes a
drop in temperature. There are two basic types of air-conditioning units; air cycle machines
(ACM) and vapor cycle systems. Large aircraft ACMs bleed compressed air from the tur­
bine engine and allow it to expand, causing cooling. With the vapor-cycle system used in
light aircraft, a pressurized liquid refrigerant evaporates, causing a temperature reduction.
This liquid refrigerant, called Freon, usually is F-21 (dichloromono-fluoroethane) or F-12
(dichlorodifluoromethane). The two, which are not interchangeable, are chosen because
they are nonflammable, nontoxic, and do not cause irritation.
The system is divided into two parts as shown in Figure 11-3. They are a high-pres­
sure side and a low-pressure side. The high side begins at the compressor discharge of
high-pressure refrigerant vapor. Driven by the engine through a belt and pulley system, a
clutch disconnects the compressor when cooling is not required. The low-pressure, lowtemperature refrigerant vapor enters the compressor, and its pressure and temperature are
raised by compression, turning it into high-pressure liquid. Then the refrigerant passes
through copper coils surrounded by cooling fins to maximize refrigerant heat transfer to
outsi e air. The condenser hangs under the fuselage in most aircraft and retracts into it
244
Environmental Systems
Evaporator
Blowers
Expansion Valve
jjg| High-Pressure
High-Pressure
‘//////. Vapor
Low-Pressure
Liquid
Low-Pressure
Vapor
Compressor
Condenser
Fig. 11-3.
Vapor cycle system.
Receiver-Dryer
Chapter Eleven
when the system is inoperative. Because of the excessive amount of drag the condenser
causes, most aircraft have a throttle interlock switch that automatically retracts it when
full power is applied, while simultaneously disengaging the compressor clutch from the
engine. After leaving the condenser, the liquid refrigerant goes to the receiver/dryer.
The receiver/dryer functions as the system’s reservoir. It contains a desicant (typi­
cally silica gel) that absorbs moisture; a single drop of water can freeze, lodge in the ex­
pansion valve, and completely stop the system! There also is another problem when
water and refrigerant mix; they form highly corrosive hydrochloric acid, which literally
will eat up the system from within. To further prevent particle blockage of the expansion
valve, a filter is installed at this point. There also is a sight glass in the receiver/dryer,
similar to the one in automobiles. With the system running, the sight glass should appear
perfectly clear; bubbles mean flow fluid level.
Next is the thermal expansion valve. This meters the refrigerant and maintains high
pressure upstream. As the liquid leaves the valve, it is sprayed through the coils, expand­
ing as it goes, assuring complete evaporation of the liquid by the end of the coils. The
valve varies refrigerant discharge, depending on the amount of heat to be removed from
the cabin. This is where the low-pressure side begins, with the refrigerant turning into a
low-pressure liquid. It is effectively the low-pressure equivalent of the condenser unit
and consists of parallel circuits of copper tubing with fins. As the hot cabin air passes
around the evaporator fins with the help of the blower, its heat transfers to the refrigerant
and continues on into the cabin as cool air. This heat raises the refrigerant temperature to
boiling, causing it to change state from liquid to vapor. One indication of insufficient
Freon is little or no cooling air and a buildup of frost on the evaporator; a hissing sound
in the evaporator is yet another indication. Completing the system, the liquid Freon now
goes back to the compressor to begin the process again.
Preflight
Preflighting the air-conditioner consists of a visual inspection of the compressor, drive
belt, pulley system, hoses, condenser inlet, and condensation outlet drain. The emphasis
should be on system integrity—primarily damage or other signs of obvious system fail­
ure. When the system is turned on initially, it should operate within one to two minutes;
otherwise, shut it down. As with all systems, it is important to read and follow the POH
carefully with respect to preflight, operation, preventive, and periodic maintenance.
As adaptable as humans have proven themselves to be over the centuries, they still
have a very limited temperature range. Within that range, there is an even smaller one
that dictates comfort and efficiency. Properly maintained environmental systems pro­
mote safe, comfortable flying.
246
12
Pressurization Systems
T
HE MAIN REASON FOR PRESSURIZING AN AIRCRAFT IS FLEXIBILITY.
Being able to select a higher altitude may give you the option of a smoother ride,
shorten your flying time, and/or provide an alternative to flying in severe weather or ic­
ing conditions. A pressurized aircraft can provide a comfortable cabin environment at
significantly higher altitudes than one that is unpressurized, in which the passengers
are required to wear oxygen masks.
Pressurization was originally developed in support of the WW-II effort and partic­
ularly for use in the high-altitude Boeing B-29 Superfortress bomber. Pressurization al­
lowed the crew to move about the cockpit and passenger compartments in relative
comfort while being able to take advantage of the benefits of high-altitude operation,
such as more favorable winds, greater wing efficiency, and turbo-supercharged engines.
Today there are a number of light aircraft that feature pressurization systems that
result in shorter flying times, lower fuel burns, higher endurance, and weather avoid­
ance. Many pilots feel that they can reap the same benefits simply by having oxygen
onboard the aircraft and breathing through individual masks, but there are some subtle
physiological drawbacks to doing so.
For one, oxygen masks are both cumbersome and uncomfortable. Using a mi
crophone with a mask is challenging. Having a simple conversation with someone
else is difficult enough, as the mask muffles what you are saying. Overall, the effect
Chapter Twelve
is to increase both fatigue and the psychological burden on the pilot. Then consider the
perspective of the nonpilot passenger who is used to flying on the airlines where they
can have a drink and breathe normally in a temperature-controlled, comfortable cabin.
Let’s face it; who wouldn’t be at least mildly uncomfortable with the idea of having to
wear an oxygen mask in such a strange environment?
Cabin pressure is maintained by “packing” air at a fairly constant flow through a
sonic nozzle and then controlling the flow of air out of the cabin. Because there is
already a constant flow of air out of the cabin through cracks, doors, window assem­
blies. and other leakage points, it is far easier to pump in more air than is necessary and
maintain the desired cabin pressure by regulating the opening of an outflow valve. Ini­
tially, manufacturing capability made leak-tight cabins impossible, but over the years
it has become evident that developing a leak-tight cabin is not only disproportionately
expensive but may not be such a good idea anyway. The constant airflow keeps cabin
air fresh.
FIXED ISOBARIC SYSTEMS
In the early days of general-aviation pressurization systems, a fixed isobaric system was
used that consisted of a primary valve and a secondary, or safety, valve. The primary
valve utilized an aneroid that was factory preset to maintain a given cabin altitude, typi­
cally 8000 feet. The safety valve, independent of the primary, was set to open under any
one of three conditions: when the cabin experienced maximum delta p, negative delta p,
or when the aircraft was sitting on the ground.
Maximum delta p represents how much cabin pressure is allowable relative to the
lower, outside ambient air pressure. It is analogous to how far you can blow up a bal­
loon safely. On start-up, once the engine is operating, there is some airflow into the
cabin. As power is increased to takeoff, the inflow rate is sufficient to pressurize
the cabin. The aircraft leaves the ground and climbs in an unpressurized mode with the
valve open. Throughout this portion of the climb the cabin pressure parallels the out­
side ambient air pressure until it reaches the fixed set point of the aneroid. As the air­
craft climbs through 8000 feet, the valve closes and maintains an 8000-foot cabin
altitude even though the aircraft continues its ascent. The aircraft maximum altitude is
limited only by the structural strength of the cabin—maximum delta p. If the aircraft
continues to climb, the valve will open up as necessary to prevent the cabin pressure
from exceeding maximum delta p. The effect under those conditions will be for the
cabin altitude to climb above 8000 feet.
Negative delta p represents a situation where the outside air pressure is greater than
the pressure inside the cabin. Negative pressure is, well, a negative situation, as the air­
frame structure is designed to contain pressure like a balloon rather than withstand outside
pressure like a submarine. Negative delta p occurs when the aircraft descends faster than
the valve can outflow cabin pressure, resulting in cabin pressure greater than ambient.
The third condition, when sitting on the ground, is important because if both valves
remained closed on the ground there could be sufficient pressure to make it difficult to
open cabin doors and/or emergency exit windows in an emergency.
248
Pressurization Systems
VARIABLE ISOBARIC SYSTEMS
Today’s light aircraft cabin pressure systems are direct descendants of the earlier fixed
isobaric systems. Instead of having a fixed system, pressurization manufacturers relo
cated the aneroids to a controller in the panel and ran pneumatic lines to the valves Turn
ing the altitude select knob on a Garrett controller simply rotates the aneroid directly
allowing more air to enter or leave the valve and causing the cabin altitude to change
Such a direct linkage does have its problems, as it makes setting cabin pressure in
flight difficult. Even a small change in dial setting can cause a rapid change because there
is no rate of change control. Newer systems incorporate a rate of change function so the
dial can be moved in flight, allowing for more flexibility.
Take, for instance, Janitrol’s pressurization system for the Cessna P210. The system
has four basic modes of operation: unpressurized. isobaric, differential, and negative re­
lief. The unpressurized mode is in effect any time the aircraft is at a lower altitude than
the cabin altitude requested by the pilot; this is common during takeoff, climb, descent,
and landing. The isobaric mode begins when the aircraft climbs through the selected
cabin altitude, which may range from below sea level to 10,000 feet. In the P210, the pi­
lot selects the desired cabin altitude on the manual controller prior to takeoff; no other
input is required through takeoff, climb, and level-off. If a change of aircraft cruise alti­
tude is required, the pilot slowly adjusts the controller to preclude abrupt cabin altitude
changes, which can be uncomfortable for passengers. Unless, of course, you don’t like
your passengers and you like that “bug-eyed” look.
The manual controller has two altitude scales, as shown in Figure 12-1. The outer
scale indicates cabin altitude; the inner scale indicates the corresponding aircraft altitude
at the maximum operating cabin pressure differential, which is the ratio between inside
and outside air pressures. These numbers on the controller face must be multiplied by
1000 feet to determine the appropriate altitude. The pilot turns the cabin rate control
knob to adjust the rate at which the cabin pressure “climbs” or “descends” to the altitude
set on the manual controller. The differential pressure mode goes into operation when­
ever the maximum cabin-to-ambient pressure differential is reached. Because differential
pressure is a measure of internal stress on the fuselage skin, if it were to become too
great, structural damage to the fuselage might occur.
The transition from the isobaric mode to the differential control is automatic. The
operating differential normally is maintained by the outflow valve with the safety valve
acting as a backup, allowing a pressure differential only slightly higher than what is reg­
ulated by the outflow valve. The reason for the slightly different pressure differentials be­
tween valves is because if it were the same on both the primary and safety valve, the two
valves would “talk” or open and close opposite of one another, which can be uncomfort­
able for passengers.
Maximum Pressure Differential
The maximum pressure differential value varies from aircraft to aircraft depending
on system and structural limitations and the type of operation tor w IC t e aircr
249
Chapter Twelve
designed. An aircraft such as the Beech Baron 58P, which has a maximum cabin dif­
ferential pressure of 3.65 pounds per square inch (psi), is limited to a difference of
3.65 psi between the ambient air pressure and the cabin air pressure. On the ground,
there is a 1:1 cabin-pressure-to-ambient-air-pressure ratio, but as the Baron 58P
climbs, that ratio changes until there is a 3.65-psi pressure difference. Table 12-1
shows that the 58P is capable of maintaining sea-level pressure up to approximately
7000 feet. At that altitude the difference between sea-level pressure and the 7000-foot
atmospheric pressure is 3.4 psi (14.7 psi – 11.3 psi), nearly the maximum pressure
differential. At 21,000 feet and maximum cabin differential pressure, the cabin alti­
tude would be approximately 10,000 feet.
Another factor that determines the maximum possible cabin pressure is the type of
pressurization system used. The higher the aircraft is designed to operate, the greater the
maximum differential needed, and the stronger the compressor output capacity required.
Turbine engines can maintain high-pressure airflow into the cabin up to very high alti­
tudes by using air from the compressor bleed-air section of the engine. Aircraft with re­
ciprocating engines use air from the compressor section of a turbocharger similar to the
system illustrated in Figure 12-2.
In light, twin-engine aircraft, powerplant failure—or even a significant, intentional
power reduction can cause the cabin altitude to rise when there is a high cabin pres­
sure differential. This can occur because the turbocharger, which is the source of the
pressurizing air, is powered by the engine. Power reductions in single-engine aircraft
ave t e same effect, so descents should be initiated far enough in advance so the
250
Pressurization Systems
power will not have to be cut back. For the same reason, pilots should be careful not to
run a fuel tank dry in a pressurized aircraft. Depending on the amount of uncontrolled
cabin leakage, cabin altitude may rise faster than you can switch tanks and get the engine running again.
CABIN AIR TEMPERATURE
When air is compressed, it increases in temperature. Turbine-engine bleed air is so
hot it always requires cooling before entering the cabin, even if warm air is desired.
Larger turbine-powered aircraft run the pressurized bleed air through either airconditioning packs or a vapor cycle air-conditioning system prior to cabin entry. Air
from a reciprocating-engine turbocharger may require cooling only on warm days
when the aircraft is flying at lower altitudes. This typically is accomplished by rout­
ing the air through a heat exchanger where the pressurized air ducting is cooled by
ambient ram air. At very cold ambient temperatures, when considerable heat is re­
quired in the cabin, the pressurized air may not be warm enough and a cabin heater
will be required.
Table 12-1
Standard Atmospheric Pressure
Altitude
(feet)
Pressure
(psi)
Altitude
(feet)
Pressure
(psi)
Sea level
14.7
18,000
7.3
1,000
14.2
19,000
7.0
2,000
13.7
20,000
6.8
3,000
13.2
21,000
6.5
4,000
12.7
22,000
6.2
5,000
12.2
23,000
5.9
6,000
11.8
24,000
5.7
7,000
11.3
25,000
5.5
10.9
26,000
5.2
10.5
27,000
5.0
8,000
9,000
10,000
10.1
28.000
4.8
11,000
9.7
29,000
4.6
9.3
30,000
4.4
13,000
9.0
35,000
3.6
14,000

8.6
40,000
2.7
15,000
8.3
50,000
1.7
16,000
8.0
17,000
7.6
12,000
251
Chapter Twelve
To Cabin Pressurization
|
| Turbocharged Induction Air
I
Exhaust
Ram Inlet Air
Fig. 12-2. Cessna 421 turbosystem schematic.
THE OUTFLOW VALVE
The outflow valve, which vents the cabin to the outside air, has three main functions:
negative pressure relief, isobaric control, and differential control. Negative pressure re­
lief is automatic, so the aircraft is never subjected to an outside air pressure greater than
cabin pressure; higher pressure air always can flow freely through the outflow valve into
the aircraft.
Isobaric control typically maintains cabin pressure within +0.05 psi from what
the pilot selects on the manual controller. If the cabin pressure exceeds that selected
on the controller, the outflow valve increases to compensate; as the pressure falls
below, the opening decreases slightly.
Differential control is preset by the factory. The isobaric pressure requested by the
pilot will be maintained until cabin pressure reaches the maximum pressure differential,
then the differential control overrides the isobaric mode so the cabin altitude will vary di­
rectly with aircraft altitude.
If the outflow valve were to stick closed, excessive cabin pressure would build up
quickly. To prevent overpressurization resulting from a stuck outflow valve, the system
has a safety valve, which functions to relieve negative pressure, to provide backup dif­
ferential control, and to act as a solenoid-operated cabin-pressure dump mechanism that
can be operated from the flight deck or by a gear squat switch.
RAPID DEPRESSURIZATION
One of the most misunderstood aspects of cabin pressurization is rapid depressurization.
Often inappropriately referred to as “explosive decompression,” movies have depicted
252
Pressurization Systems
scenes where a single bullet shot through the fuselage has sent people flying about the
cabin. It makes for high drama but it is pure nonsense. If you can have a fairly large out­
flow
valve open to the atmosphere, what difference would a small bullet hole make” As
long as the bullet hole remains small, the outflow valve can compensate instantly
A pilot may depressurize the cabin intentionally if the pressurization system begins
to pump contaminated air into the cabin. Also, if the pilot discovers a window is crack­
ing, depressurization will relieve the pressure differentia] and take stress off the window.
There are two ways to depressurize a cabin intentionally. The gentlest would be to
increase the cabin altitude slowly with the manual controller until there is no longer a
pressure differential; this would be an appropriate method in the case of a cracked win­
dow, limited smoke, or fumes. In a life-threatening situation, such as a cabin fire or dense
smoke, the pilot could elect to activate the depressurization switch. This reduces the pres­
sure differential to zero rapidly, but not instantly; the outflow and safety valves are not
that large!
One of the frequent causes of rapid decompression is the improper closure of a door.
Unlike larger aircraft with plug doors that open into the cabin and are pushed into the fuse­
lage by cabin pressure, light aircraft doors generally open outward, away from the fuselage.
The manufacturer has to devise a locking mechanism that is easy to use but strong enough
to ensure the door will not open in flight.
If someone incorrectly locks the door, it could
blow out when the pressure differential increases. The potential for this problem, which is
almost always caused by human error, is significant enough in the King Air 200 that when
I worked for FlightSafety we used to instruct all crews never to permit passengers to lock
the cabin door; it was to be locked only by a crewmember. This is good advice for all pres­
surized aircraft. If it isn’t intuitively obvious, the reason that light aircraft doors open out­
ward is simply because of the limited room inside the cabin.
Even if a door or window did burst open in flight,
it isn’t very likely that passengers
of light aircraft are going to be sucked out of their seats and pulled through it. An anal­
ogy would be if you filled
the airplane with water and punched a hole in the fuselage; the
bigger the hole, the more the water would tend to pull you toward it. Again, this is pre­
dominantly high drama in films;
however, when flying
as a passenger I find myself, tor
some inexplicable reason, never choosing a seat next to a door!
There should be concern for people who have ear or sinus trouble, however, because
instantaneous pressure changes can be painful. Cold, fear, and lack of oxygen present a
far more serious problem than the rapid decompression itself. It feels as if someone has
stepped on your chest as the air suddenly expands in your lungs, forcing its way out
through your nose and mouth. You couldn’t hold your breath it you tried, but it wou
never occur to you to try. The cabin develops a condensation cloud for a short time, ma
ing it difficult, if not impossible, to see within the cabin. This could be a s ort
ura
problem for the pilot, as instruments could become difficult to see. Very quic y a er
compression it gets incredibly cold, which becomes a major concern,
anic co
problem for passengers with heart trouble.
mrr*nt immediLosing cabin pressure when flying above 10,000 feet proba y vw ^ ^ ^ ^
available, the cold can be deadly. The best
ate descent. Even with supplemental oxygen
253
Chapter Twelve
course of action in that event is as follows. First, fly the aircraft! Nothing else matters if
you lose control of the aircraft. Second, don an oxygen mask and make sure you have
oxygen flowing. Third, pause for just a second to shake off the fear, then check the pas­
sengers to make sure they are on oxygen. Finally, begin a descent while assessing the sit­
uation and determine the best course of action based on cabin temperature, oxygen
available, structural condition of the aircraft, weather, distance to the airport, wind con­
ditions at a lower altitude, and the condition of the passengers. Don’t underestimate the
physiological effect of high-altitude, unpressurized operation on you or your passengers.
Your best insurance policy in this type of situation is to have taken the time in advance to
be familiar with applicable emergency procedures in the pilot’s operating handbook.
Also, be aware that a lower altitude frequently means increased turbulence, so descent
should be at a reasonable speed, not at Vne.
Another concern about rapid descent is existing structural damage because the airframe
may not hold up under high-speed descent. Also remember that as you descend, air density
increases and so will the indicated airspeed for a given deck angle.
There are other concerns related to cabin depressurization. If sitting in the freezing
cold contemplating a course of action sounds like a great time for a cup of hot coffee, re­
member that the thermos was sealed at ground pressure, so it is a potential bomb. Don’t
forget to squawk 7700 on the transponder. If you are in instrument conditions and it be­
comes necessary to use alternate instrument air, be aware that a large hole in the fuselage
can cause the cabin pressure to be lower than ambient due to a venturi effect. Perhaps
most often overlooked is a passenger briefing before the flight;
a little knowledge can go
a long way, especially concerning the use of oxygen and the effects of smoking.
PREFLIGHT AND OPERATIONAL CONSIDERATIONS
During the preflight, make sure the door is properly sealed and the dump switch is off.
After engine start, to assure the system will work while still on the ground, set the aircraft
altitude controller to 500 feet below field elevation. Now pull the landing gear circuit
breaker and increase the rate controller; the system should begin to pressurize the cabin
because you have overridden the gear squat switch and tricked it into “thinking” it was
flying above the selected altitude. Then test the dump switch to make sure it will work if
you should need it in flight. You should never take off in a pressurized condition because
the aircraft is not designed for it.
In preparation tor takeoff in the Cessna 340, which uses the Garrett AiResearch sys­
tem, the procedure is somewhat different than in the Cessna 210. AiResearch instructs pi­
lots to select 500 feet above field elevation on the cabin altitude selector and set the cabin
rate control knob to the 12 o’clock position. Then start the engines and check for airflow
into the cabin to assure it will pressurize after takeoff. There are two reasons for doing
this: First, it prevents the pressure “bump” sometimes felt on takeoff as a result of both
the safety and outflow valves closing simultaneously. The safety valve, which closes
when the gear retracts, is controlled by the squat switch. The outflow valve closes when
the cabin reaches the altitude you have requested. If the controller is set to field elevation,
both may slam shut simultaneously on takeoff. With the controller set to 500 feet above
254
Pressurization Systems
field elevation, the outflow valve wtll close long after the safety valve and the passenm,
will experience a smoother transition. The second reason is that if you set the selector m
cruise altitude, the system will “prerate,” meaning it will think the aircraft is climbin.
long before it actually does. This will delay normal cabin pressurization longer than nec
essary and may prove uncomfortable for some passengers.
Once the climb is established and you have passed through 500 feet AGL reset the
aircraft altitude selector to 1000 feet above cruise altitude. As the aircraft climbs the
cabin altitude takes care of itself. The reason for setting 1000 feet above cruise altitude is
again passenger comfort. If the controller is set to cruise altitude, the outflow valve will
open and close continuously as the cabin pressure makes small fluctuations between too
low and too high. With the controller set for an altitude above the actual aircraft altitude,
the cabin will never reach the programmed pressure, so the outflow valve will remain at
least slightly opened, the result is no bumps. There are no additional requirements for
cruise condition. If it is necessary to change altitude, simply select the new altitude plus
1000 feet and climb or descend.
During descent for landing, set the aircraft altitude selector to approximately 500
feet above field
elevation and adjust the cabin rate of change to maintain a comfortable
cabin rate of descent. It is a good idea to not descend at a rate that will allow the aircraft
to catch up to the cabin altitude, otherwise the cabin will depressurize. On the other
hand, if you select the field
elevation for the cabin altitude, when the gear touches down
on the runway the cabin will dump, causing some passenger discomfort. With 500 feet
above airport elevation selected, the cabin will depressurize comfortably shortly before
landing.
TROUBLESHOOTING
Fortunately, pressurization systems are basically reliable, but things can go wrong. Here
are a few thoughts on troubleshooting.
If there is a “bump” felt at rotation on takeoff, an ear-popping event caused by a sud­
den pressurization, it is probably the result of the outflow valves closing too rapidly,
fuselage flex, or a change in airflow over the outlet holes. Normal fluctuations in pres­
sure, depending upon their frequency and magnitude, may not be felt by the occupants,
but bumps are usually uncomfortable and can potentially cause structural damage.
If the cabin follows the aircraft’s altitude and rate-of-change shown on the flight in­
struments, there may be several causes. The first thing to check is the dump switch. If the
dump switch is off, the logical choice would be a problem in the landing gear solenoid
valve, which is responsible for keeping the safety valve open during ground operations.
Try cycling the gear to see if that helps, then try opening the landing gear circuit breaker
to bypass the system.
If the down rate is faster than the up rate, but everything else works norma y. just
make the necessary adjustment manually and have the controller checked out at your
next opportunity. The problem probably is a minor leak in the tubing or contro er.
Should the cabin rate exceed the selected rate value during the aircraft s climb o cm
altitude, increase the rate selection or decrease the aircraft s rate o c lm
255
Chapter Twelve
answer is that the aircraft is climbing faster than the controller and it is at the maximum dif­
ferential; however, it also could be a controller malfunction.
If the cabin altitude exceeds what you have selected, one of several problems may
exist. There may be a loss of pressurizing airflow for some reason. The aircraft altitude
may have exceeded the positive differential pressure value. There may be an internal
malfunction of the controller, the outflow valve, or safety valve. Or you may have some­
thing as simple as a leak in the tubing. In any event, your only choices would be either to
adjust to a higher cabin selection, if possible, or to reduce the aircraft altitude. Other
problems that may arise are usually beyond the pilot’s ability to correct in flight.
256
10
Landing Gear Systems
– E
T H p
SYSTEMS ON AN AIRCRAFT, LANDING GEAR CAN
0certainly be the most embarrassing. It^ te
damage by the pilot simply forgetting to move
into thinking the landing gear warning horn will prevent y
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^”pUof-whosncce^Uygo^^neE^^
after the engine had failed some distance a y P
tensive damage to an otherwise unblemis e air ^
the inquiry he was asked
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how such a thing could happen after Ihut successf
he was under enormous stress
in the crippled aircraft. He told the board of’inq ry
as he ap.
and couldn’t think clearly because of a loud hom blasting
proached the runway. It was the gear-up warning
vunpht Flyer used skids and a
Interestingly, landing gear was an
rail system. Dr. Samuel Langley s ill-fated A
Jbe launched from
^ ^ watgr putting tires
a houseboat floating
down the Potomac River
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