Environmental Systems practice questions

Answer: B) A crack can let exhaust carbon monoxide mix with the heated cabin air

Exhaust-shroud heaters warm cabin air by passing it over the exhaust muffler, so any crack can carry carbon monoxide into the cabin. For that reason FAA AC 91-59A stresses close inspection of heater muffs and heat exchangers for cracks and leaks.

• AIncorrect. The shroud wraps around the exhaust muffler, not the induction system, so a crack there does not let in fuel vapor. The danger is exhaust gas.
• BCorrect. Fresh air is heated by flowing over the exhaust inside the shroud. If the muffler or heat exchanger cracks, exhaust gas, including carbon monoxide, can be carried into the cabin through the cabin heat system (AC 91-59A).
• CIncorrect. A muffler crack does not cause an engine overspeed. The safety concern with heater-muff cracks is carbon monoxide getting into the cabin.
• DIncorrect. Heater air leaking into the exhaust after combustion does not change the fuel-air mixture going into the cylinders. The real hazard runs the other way: exhaust gas getting into the cabin air.

Answer: D) About 10,000 feet

• AIncorrect. Cabin altitude warnings are standard.
• BIncorrect. That is too low.
• CIncorrect. That is far too high to wait.
• DCorrect. The warning alerts the crew to loss of pressurization.

Answer: A) It sets how quickly cabin pressure altitude is allowed to rise or fall

The rate selector controls how fast cabin pressure altitude changes, which is separate from the target cabin altitude and the maximum differential. Its purpose is occupant comfort.

• ACorrect. The rate control limits how fast cabin altitude changes, typically by controlling how quickly the outflow valve moves. This prevents uncomfortable pressure changes on occupants' ears and sinuses.
• BIncorrect. The maximum differential is a structural limit enforced by the controller's differential function and by the safety (pressure relief) valves. The rate selector does not set it.
• CIncorrect. That is the job of the cabin altitude selector, which sets the target cabin altitude, not how fast the cabin gets there.
• DIncorrect. The pressurization controller has no control over the aircraft's own climb or descent rate. It acts only on cabin pressure, through the outflow valve.

Answer: B) It stores refrigerant and removes moisture

• AIncorrect. That is the compressor.
• BCorrect. Moisture can freeze and damage the system.
• CIncorrect. That is the evaporator.
• DIncorrect. It is not a heater.

Answer: B) Positive pressure relief (safety) valve

Pressurized cabins have positive pressure relief (safety) valves that open automatically to limit differential pressure if the controller or outflow valve fails. Negative pressure relief valves protect against the opposite condition.

• AIncorrect. A negative pressure relief valve opens when outside pressure becomes higher than cabin pressure, for example during a rapid descent. It does not guard against too much positive cabin pressure.
• BCorrect. The positive pressure relief or safety valve works independently of the controller and outflow valve. It opens automatically to keep the cabin-to-outside pressure differential from going over its preset maximum.
• CIncorrect. The cabin altitude limiter closes the outflow valve to stop cabin altitude from climbing too high, which means cabin pressure falling too low. It acts in the opposite direction from what is needed here.
• DIncorrect. The squat-switch circuit keeps the cabin depressurized while the aircraft is on the ground. It gives no overpressure protection in flight.

Answer: B) Chemical oxygen generators mounted with the mask units

Many airliners supply drop-down passenger masks from chemical oxygen generators, which a chemical reaction starts once the mask is pulled. Others use central gaseous oxygen cylinders feeding passenger supply lines.

• AIncorrect. Liquid oxygen converters are mostly found on military aircraft. They are not the usual source for airliner passenger drop-down masks.
• BCorrect. On many airliners each group of masks has its own chemical oxygen generator. Pulling a mask down starts a chemical reaction that produces breathing oxygen (14 CFR 25.1450 defines these generators). Some airliners instead feed the masks from central gaseous oxygen cylinders through supply lines.
• CIncorrect. Molecular-sieve oxygen concentrators (OBOGS) mainly supply crew oxygen on military aircraft. They are not the standard source for airliner passenger drop-down masks.
• DIncorrect. Portable bottles are for cabin crew moving around and for first aid. They are not plumbed to the automatic drop-down passenger masks.

Answer: A) They capture very fine airborne particles, including bacteria and viruses, from the recirculated air

HEPA filters in the recirculation path capture about 99.97% of particles 0.3 micrometers and larger, including bacteria and viruses. The filtered air is then mixed with outside air before it returns to the cabin.

• ACorrect. Recirculated air passes through HEPA filters that capture about 99.97% of particles 0.3 micrometers and larger, such as bacteria, viruses and fungi. This reduces the airborne contaminants returned to the cabin.
• BIncorrect. HEPA filters are particulate filters. They don't remove gases such as ozone. Ozone is handled by separate ozone converters in the outside-air supply.
• CIncorrect. A filter removes particles. It doesn't add moisture to the air.
• DIncorrect. Only part of the cabin air (about 35–55%) is recirculated, and it is always mixed with outside air. HEPA filtration doesn't remove the need for an outside-air supply.

Answer: B) Rejects heat outside

• AIncorrect. That is the evaporator.
• BCorrect. The condenser releases heat from the refrigerant to outside air.
• CIncorrect. This is unrelated.
• DIncorrect. This is unrelated.

Answer: B) Releases all cabin pressure quickly

• AIncorrect. It reduces pressure.
• BCorrect. It is used on the ground or in emergencies.
• CIncorrect. Bleed air valves do that.
• DIncorrect. Mixing valves do that.

Answer: A) Bleed air expands across a turbine

• ACorrect. Expanding compressed air through a turbine lowers its temperature.
• BIncorrect. That is a vapor cycle system.
• CIncorrect. This is not how it works.
• DIncorrect. This is dangerous and wrong.