Question: Can you tell me if there is backup power for every crucial device on an aircraft, i.E. The landing gear, flaps, rudder and all other important parts of the plane, in the event of power failure?
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— Submitted by reader David, U.K.
Answer: There are redundant systems for all critical systems. As an example, there is a backup to extend the landing gear if the primary hydraulic system fails. Flaps and flight spoilers have backup systems too. Some items, such as ground spoilers may not have a backup; in those cases the pilots calculate the landing distance with the inoperative component to ensure the runway is long enough.
Pilots practice landing with inoperative systems in the flight simulator to ensure their familiarity with the procedures.
Q: Capt. Cox, aboard a 737 at DFW, the pilot advised us after landing that the outside air temperature was about 105º and recommended everyone pull down their window shades and open overhead air nozzles to keep cabin temperatures down because the plane’s APU, which would normally provide climate conditioning at the gate, was out of order. I was shocked to realize that our aircraft was considered airworthy without a functional APU. It’s not that I care about the A/C comfort levels…It’s about the Miracle on the Hudson, when Capt. Sully had to rely on his APU to keep control of the aircraft. Is it true that the APU is considered a non-critical item in the airworthy checklists?
— Sandy, Los Angeles.
A: Every airplane has a Minimum Equipment List (MEL) that lists all the systems or components that may be inoperative for a flight. The MEL also states restrictions that would apply to a flight with an inoperative component.
Determination of which components are allowed to be inoperative utilizing the MEL, the restrictions, and the duration that a component is permitted to be inoperative are the result of meetings with the operators, manufacturers, FAA, and often pilot union representatives. This process has a long history of success.
Inoperative items utilize the built-in redundancy in modern airplanes. As an example, a 737 or A320 has three generators onboard (one on each engine and one attached to the APU). It is permissible to fly for a few days with one generator inoperative, provided the other two are operating. This allows the flight to be on-time without any compromise in safety. If, during the time when one generator is inoperative, another generator fails, a landing at the nearest suitable airport is required.
Airlines have safely utilized the MEL for decades. I have been involved in MEL meetings to determine the restrictions for inoperative components for the B737. The professionalism of the experts is very high and I support their decisions.
Your example of US Airways 1549 overlooks the fact that when the second engine failed, the Ram Air Turbine would provide backup electrical and hydraulic power. While not as capable as the APU, it would have provided power for flight instruments for the captain and flight-control computers and hydraulic power for the flight controls. I think Captain Sullenberger would have had a similarly successful landing if he were flying on power from the Ram Air Turbine rather than the APU.
John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.