High Altitude Operations

Question 1

Why do you think it’s important for pilots to have the concepts in FAR 91.211 - supplemental oxygen requirements - committed to memory?

Answer 1

Because ATC generally has no idea whether my aircraft has supplemental oxygen or even what the 91.211 regulations require, meaning ATC often issues climb instructions that pilots must be prepared to reject when flying aircraft that are not equipped with supplemental oxygen.

Question 2

During a day flight, you are told to climb to 12,500ft and that you can expect to be at that altitude for the next couple hours. Can you accept?

Answer 2

Legally yes. The oxygen requirements are above- type requirements, meaning each additional restriction kicks in above the associated cabin pressure altitude threshold. So for instance, the requirement for crew to use supplemental oxygen kicks in when flying above a cabin pressure altitude of 12,500ft for more than 30 minutes. So 12,500 would be the highest VFR cabin pressure altitude permitted for operation without supplemental oxygen.

Question 3

During a day flight, you are told to climb to 13,500ft and that you can expect to be there for 15 minutes. Can you accept?

Answer 3

Legally yes, as long as the time spent above 12,500ft does not exceed 30 minutes. Good ADM would likely require an “unable” reply, though, as it will take significant time to climb to 13,500 and descend back to 12,500, and the 15 minutes is just ATC’s projection.

Question 4

During a night flight, you are told to climb to 13,500ft and that you can expect to be there for 15 minutes. Can you accept?

Answer 4

“yes”; however, the FAA recommends the use of supplemental oxygen at night when operating at altitudes above 10,000ft during the day and 5,000ft at night.

Question 5

Why does the FAA recommend oxygen at such low altitudes at night?

Answer 5

Mostly because night vision tends to degrade rapidly above a 5,000ft pressure altitude. Eyes use rods at night, and rods are much more sensitive to oxygen deprivation than cones, which we use to see during the day. Also, pilots are generally more fatigued when flying at night, and fatigue aggravates the effects of hypoxia.

Question 6

Specifically, what are the general supplemental oxygen requirements?

Answer 6

Required to be used by the required flight crew above a 12,500ft cabin pressure altitude up to 14,000ft when at those altitudes for more than 30 min. Required to be used by the required flight crew at all times above a cabin pressure altitude of 14,000ft. All occupants must be provided with oxygen above a cabin pressure altitude of 15,000ft.

Question 7

When you say “cabin pressure altitude,” do you mean that if you are operating with an altimeter setting below standard, say 29.72, at a flight altitude of 12,500ft MSL for more than 30 minutes, that you would be required to use supplemental oxygen? Because the pressure altitude in the cabin is above 12,500?

Answer 7

Yes.

Question 8

Are these general supplemental oxygen requirements for all aircraft or only those without pressurized cabins?

Answer 8

All. E.g. if the cabin pressure altitude on a pressurized aircraft were to exceed 12,500ft for more than 30 min, the required crew would have to use supplemental oxygen, etc.

Question 9

Take me through the supplemental oxygen requirements that apply to aircraft with pressurized cabins.

Answer 9

The general requirements still apply. In addition, at least a 10 minute supply of supplemental oxygen must be available for each occupant for flights above FL250. Above FL350, at least one pilot at the controls needs to be using supplemental oxygen; however, there is an exception to this rule if quick donning masks are available: as long as there are two pilots at the controls, neither needs to be using supplemental oxygen. This exception goes away above FL410.

Question 10

What qualifies as a quick donning mask?

Answer 10

Basically a mask that can be secured on the face with one hand and start supplying oxygen within 5 seconds.

Question 11

What is the major risk of flying at an altitude above 10,000ft without using supplemental oxygen?

Answer 11

Hypoxic hypoxia.

Question 12

What else might lead to hypoxic hypoxia, other than flying a non-pressurized aircraft above 10,000ft without supplemental oxygen?

Answer 12

A rapid or explosive decompression event, a pressurization system malfunction, or an oxygen system malfunction.

Question 13

On a physiological level, what causes hypoxic hypoxia when flying at high altitudes without supplemental oxygen?

Answer 13

The partial pressure of oxygen is so reduced at higher altitudes that when this low-pressure oxygen gets delivered to the lungs, the lungs can’t use it - i.e. the lungs can’t transfer the oxygen from the ambient air to the blood to be carried throughout the body.

Question 14

What kind of symptoms would you expect from hypoxic hypoxia?

Answer 14

Cyanosis (blue fingernails and lips), inflated sense of well being or euphoria, headache, decreased response to stimuli/increased reaction time, impaired judgement and alertness, visual impairment, drowsiness, lightheaded or dizzy sensation, tingling in fingers and toes, numbness, tunnel vision.

Question 15

Other than fatigue and night flying, what can aggravate the effects of hypoxia?

Answer 15

Smoking, alcohol, drugs, poor physical fitness or an underlying medical condition like anemia, certain over-the-counter medications, as well as any situation that increases the body’s demand for oxygen, such as extreme heat and cold, fever, and anxiety.

Question 16

What are the 3 components of most oxygen systems?

Answer 16

Storage system (containers), delivery system, mask or nasal cannula.

Question 17

On a commercial airline flight, what type of oxygen delivery system would you expect the aircraft to provide for passengers?

Answer 17

Continuous flow.

Question 18

What type of oxygen delivery system uses the dixie cup

Answer 18

Continuous flow.

Question 19

How does the dixie cup system work?

Answer 19

A continuous flow of oxygen is delivered into a rebreather bag. The passenger breathes in this oxygen through an oral/nasal cup or an airline drop-down unit (dixie cup). Exhaled air is released to the cabin.

Question 20

Continuous flow systems are considered effective up to approximately what altitude?

Answer 20

25,000ft.

Question 21

You’re cleared for a climb to FL350. What type of oxygen delivery system(s) would be necessary for the pilots?

Answer 21

Diluter demand or pressure demand.

Question 22

Describe how a diluter demand system works.

Answer 22

Oxygen is delivered only when the user inhales, thus the “demand” part of the name. At lower altitudes some of the supplemental oxygen that the user inhales is diluted with outside air. As the altitude increases, the oxygen becomes less and less diluted, eventually becoming 100% pure oxygen.

Question 23

Up to what altitude is the diluter demand system considered effective?

Answer 23

40,000ft.

Question 24

You’re cleared for a climb above FL400. What type of oxygen delivery system is necessary for the pilots?

Answer 24

Pressure demand.

Question 25

Describe how the pressure demand oxygen system works.

Answer 25

The only difference between pressure and diluter demand systems is that pressure demand supplies the oxygen to the mask and lungs under pressure. This makes pressure demand safe to use above 40,000ft, where 100% oxygen without positive pressure will not suffice.

Question 26

What type of oxygen are pilots required to use, and why?

Answer 26

Aviator’s Breathing Oxygen, because it abides by the Aviator’s Breathing Oxygen Purity Standard, which requires at least 99.5% pure oxygen (in practice the oxygen is virtually 100% pure, as are medical, welding, and research oxygen).

Question 27

During a preflight, how can pilots verify that the oxygen in the supplemental oxygen container(s) is Aviator’s Breathing Oxygen?

Answer 27

It’ll be labeled as such.

Question 28

Why does the FAA advise against using medical grade oxygen?

Answer 28

Because of the potential for water molecules that can freeze in low temperature environments and clog the oxygen delivery lines.

Question 29

What precautions should be taken when it comes to servicing, handling, and storing oxygen? Why?

Answer 29

Not only is oxygen flammable, but it renders materials combustible that are normally nearly fireproof. Because of this, oils and greases should not be stored in the vicinity of the oxygen, nor should they be used to seal the valves and fittings of oxygen equipment. Also, smoking during the use or servicing of any kind of oxygen equipment is also strictly forbidden. When it comes to servicing the oxygen, the FAA advises marking the bottle with the recommended pressure (usually 1,800-2,200psi) before filling the tank to that pressure level.

Question 30

What checklist can pilots use to ensure a proper preflight check of the oxygen system? Describe it.

Answer 30

PRICE: Pressure (ensure that there is enough oxygen pressure and quantity to complete the flight), Regulator (inspect the oxygen regulator for proper function), Indicator (ensure that the flow indicator shows a steady flow of oxygen when in use), Connections (make sure all the connections are secured), Emergency (ensure sufficient oxygen for any emergency that might occur during the operation. This step should include briefing the passengers on the location of oxygen and its proper use).

Question 31

What are some of the main reasons that aircraft fly at high altitudes?

Answer 31

To avoid bad weather/turbulence, avoid terrain, fuel efficiency (an aircraft flown at high altitudes consumes less fuel for a given airspeed than it does for the same speed at a lower altitude), and to fly faster groundspeeds.

Question 32

What prevents occupants from being hypoxic all the time when operating at high altitudes?

Answer 32

Cabin pressurization or supplemental oxygen.

Question 33

Let’s say you just passed your Private Airplane Multi-Engine Land practical test. Can you immediately jump in a multi-engine pressurized airplane and pilot it?

Answer 33

More than likely I’ll likely need a high-altitude endorsement. Plus, most pressurized airplanes require a type rating, so I’ll probably need one of those as well.

Question 34

Is this high-altitude endorsement always required to fly pressurized aircraft?

Answer 34

No it’s technically only required if the aircraft is pressurized AND has a max operating altitude or service ceiling above 25,000ft MSL, whichever is lower.

Question 35

Do you have to carry this endorsement on you (so carry your logbook) when you fly pressurized aircraft?

Answer 35

No.

Question 36

Where are the requirements to receive the high altitude endorsement listed?

Answer 36

61.31(g)

Question 37

How does a basic pressurization system work?

Answer 37

Pressurized air from the compressor section of the engine is allowed to bleed into the aircraft. A heat exchanger cools the hot compressed air before it enters the cabin. Then valves onboard control the exit of this pressurized air through a controlled leak - the valves either close in order to trap the pressurized air inside the airplane and lower the cabin pressure altitude, or they open to let the pressurized air escape in order to increase the cabin pressure altitude. This process is all controlled automatically by a cabin pressure regulator .

Question 38

Name and describe the functions of these valves (each pressurized aircraft has its own unique system - just describe the generic valve system detailed in the PHAK).

Answer 38

The outflow valve(s) is the primary pressure-regulating valve that opens and closes in order to regulate how much pressurized air exits the aircraft; this allows for a constant inflow of high-pressure air to the pressurized areas of the aircraft, followed by a controlled leak through the outflow valve(s) back out to the atmosphere. The safety valve is a combination pressure relief, vacuum relief, and dump valve (in practice these are often seperate valves). The pressure relief component prevents cabin pressure from being too high in relation to that of the ambient air pressure (provides redundancy for the outflow valve). The vacuum relief component prevents ambient air pressure from exceeding cabin air pressure by allowing external air to enter the cabin in the rare event that ambient pressure exceeds cabin pressure. The dump valve aspect just dumps all the cabin air into the atmosphere.

Question 39

What is a scenario where vacuum relief might be necessary - i.e. when would ambient air need to be let INTO the cabin to prevent a situation where the air pressure inside the cabin is lower than outside?

Answer 39

Rapid or emergency descents.

Question 40

What’s so bad about the air pressure inside the cabin being lower than outside?

Answer 40

The walls of the aircraft are built to expand and withstand low external pressure/high internal pressure situations, not the other way around.

Question 41

When would it be necessary to use the dump valve?

Answer 41

When there is a need to immediately depressurize an aircraft during an emergency such as a cracked window, contaminated bleed air, a cabin fire that can be controlled or extinguished by starving it of oxygen, or the need to clear the cabin of smoke.

Question 42

What prevents the compressed air from being hot?

Answer 42

The compressed air goes through some sort of heat exchanger before being ducted into the cabin.

Question 43

What other function(s) does a pressurization system serve?

Answer 43

Removes odors in the cabin by keeping fresh air circulating; prevents excessive pressure differentials between the air inside and outside of the cabin (which can weaken or damage the structural integrity of the airplane); prevents altitude sickness and other high-altitude related physiological conditions among occupants; moderates the rate of pressure change inside the cabin for occupants’ comfort; keeps belongings/baggage from becoming damaged.

Question 44

What maximum cabin pressure altitude do most airline pressurization systems maintain?

Answer 44

8,000ft.

Question 45

Are baggage compartments pressurized as well?

Answer 45

Yes.

Question 46

The pressurization system fails during a flight at 40,000ft. How long until your performance becomes impaired?

Answer 46

Immediately. Although my Time of Useful Consciousness (TUC) would be a bit longer.

Question 47

Does TUC refer to the onset of unconsciousness?

Answer 47

No. Impaired performance may be immediate.

Question 48

So how do you define TUC?

Answer 48

This is the period of time from exposure to an oxygen-poor environment, to the time when an individual is no longer capable of taking proper corrective and protective action.

Question 49

What TUC would you expect at 40,000ft?

Answer 49

Approximately 15-20 seconds.

Question 50

What TUC would you expect at 20,000ft and 30,000ft?

Answer 50

5-12 minutes or more for 20,000ft (duration varies among the FAA sources); 1-2 minutes at 30,000ft.

Question 51

How precise are these TUCs?

Answer 51

Not very. They vary according to each person’s sensitivity to oxygen deprivation, as well as according to how rapidly pressurization is lost. Rapid decompression, for instance, greatly reduces these TUCs.

Question 52

If a rapid decompression situation occurs, by how much would you reduce the expected TUC?

Answer 52

50% (per AC61-107).

Question 53

A cabin window just blew out at FL350. What type of decompression would you expect?

Answer 53

Depends on the size of the airplane and the capabilities of the pressurization system. A very small jet would decompress explosively or rapidly, whereas a large jet might actually be able to maintain some degree of pressurization.

Question 54

If the pressurization system slowly fails, what kind of decompression would occur?

Answer 54

Gradual (aka slow).

Question 55

A large chunk of the roof of the airplane just blew off at FL350. What kind of decompression would occur?

Answer 55

Explosive.

Question 56

What distinguishes explosive from rapid decompression?

Answer 56

The primary difference is that during explosive decompression, the cabin pressure decompresses faster than the lungs; in rapid decompression, the lungs are able to decompress before the cabin.

Question 57

Time-wise, what is the cut-off between rapid and explosive decompression?

Answer 57

Any decompression that occurs in less than .5 seconds is considered explosive.

Question 58

Besides the potential for getting sucked out of the aircraft, what makes explosive decompression so dangerous?

Answer 58

When the air inside your lungs cannot decompress before the ambient air, the air inside your lungs gets pulled out with enough force to potentially cause lung damage. Dangerous flying debris will also be more likely.

Question 59

How would you expect the cabin environment to change after a rapid or explosive decompression event?

Answer 59

There would likely be noise, fog, flying debris, dust, wind blast, cooler temperatures, and gas expansion in the body.

Question 60

Why is there fog after a rapid or explosive decompression event?

Answer 60

The ambient air rushing into the cabin causes a rapid temperature drop and thus a change to the relative humidity.

Question 61

Which type of decompression is the most dangerous, and why?

Answer 61

The AC says that “ slow decompression is as dangerous as or more dangerous than a rapid or explosive decompression.” This is because “by its nature, a rapid decompression commands attention. In contrast, a slow decompression may go unnoticed and the resultant hypoxia may be unrecognized by the pilot.”

Question 62

What is the best indication of gradual decompression?

Answer 62

Automatic visual and aural warning systems generally provide an indication of slow decompression. In addition, having experience in an altitude chamber allows the occupant to more immediately recognize signs and symptoms of hypoxia.

Question 63

What are the dangers associated with decompression?

Answer 63

Hypoxic hypoxia is the primary one. Another danger is evolved gas decompression sickness, wind blast, being struck by debris, and being tossed or blown out of the aircraft if located near the opening(s).

Question 64

What are some of the associated cabin instruments that you would expect to see on an aircraft with a pressurization system? What do they measure/indicate?

Answer 64

Cabin rate-of-climb indicator, cabin altimeter, and a cabin differential pressure indicator. The cabin rate-of-climb indicator shows the rate-of-climb and descent of cabin pressure; the cabin altimeter shows the pressure altitude of the cabin; the cabin differential pressure indicator shows the difference in air pressure between inside and outside of the aircraft.