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I am curious about the redundancy they provide in modern era commercial aircraft. Here are the questions:

  1. If an aircraft suffers partial or complete software malfunction, do the pilots have the choice of flying the craft with help of analog consoles for critical systems(e.g. speed, altitude, heading indicators and basic flight controls). It summarizes to the point whether they provide such analog devices ?

  2. In case of such failure, can the engines keep running so as to provide thrust, fuselage pressurization and breathable air supply to cabin?

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I read somewhere that for the A320 in case of complete electrical failure the trim wheel can be used for basic pitch control. You may also consider flight control describe in this wikipedia page –  Manu H Jun 19 at 14:07

2 Answers 2

up vote 21 down vote accepted

All commercial aircraft have some form of redundancy in their instrumentation, but it's not always in the form of "analog" instruments. The backup is often electronic itself. But the overall system is designed with a very high level of redundancy.

A typical modern jet aircraft has an Electronic Flight Instrument System (EFIS), which is the large screen Primary Flight Display (PFD) you see in front of both pilots. There are usually also one or more Multifunction Displays (MFDs) in the middle between the pilots. These displays form the primary instrumentation.

The EFIS itself is designed with a very high level of redundancy between the pilot's and copilot's side displays. Both are on independent electrical circuits and receive information from independent input computers (air-data [ADC], attitude/heading [AHRS], etc.). Both can usually also be switched over to the other set of input computers. Usually, the MFD can also be configured in a "reversionary" mode where it looks and acts like a PFD.

If the entire EFIS system should fail, there is also a backup set of instruments. In modern times this backup is usually one or two Electronic Standby Instrument Systems (ESIS). These are essentially smaller, condensed "PFDs" that display critical information. They usually either contain their own miniature AHRS/ADCs or are connected to isolated computers. They are also powered on a totally independent electrical circuit and often have their own dedicated battery as well, separate from the main aircraft battery.

Modern engines are controlled by completely different computers called Full-Authority Digital Engine Controllers or FADECs. Two or more FADEC channels are provided for each engine, so that if either channel fails, the other can take over. FADECs are also considered flight-critical items, and are designed with fail-passive capability: even if both FADEC channels fail, the engines can still run and be controlled more primitively and less efficiently.

Electronic pressurization controllers don't always have redundant computer modules, but a manual pressurization control mode is always provided. In the worst case, the pilot has the ability to manually open or close the cabin outflow valve to control cabin pressure. And redundant pressurized air sources are available from each engine.

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Surprisingly, whilst most combat aircraft are jam packed with fall back analog instrumentation, despite often being aerodynamically unstable and requiring fly by wire to actually fly (SU-27 for example), many modern airliners do not even though they are actually stable and can be flown manually (Go figure!). They do however, have multiple resilient systems as stated by dvnrrs above, and importantly, pilots use (Or should use) a "cross-check" in the event of instrument failure. So, if for example IAS is lost, then ground speed, engines, pitch, etc. should be used instead.

This subject has been discussed in depth, and with massively serious consequences since the French Airbus disaster Flight 447. Reports of flight 447 show that airspeed information was lost as one of the critical factors leading up to the accident. Increased pilot training and systems will be coming I think to help mitigate future failings like occurred on that tragic day over the Atlantic ocean.

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The Airbus in Flight 447 had redundant airspeed sensors which all got iced over. One major problem with that flight was, like you mentioned, the inability of the flight crew to cross-check (there was plenty of indication of stall, such as an excessive rate of descent). Fly-by-wire and computer assisted controls are not one and the same, however (all Airbus planes are fly-by-wire). The unstable design of all modern fighter jets is due to the designers' desire to make those aircraft highly maneuverable, fly-by wire is simply a means to an end. –  alexsh May 14 '14 at 22:44
Thanks to the fine work done by TsAGI, the Su-27 is stable over its whole angle of attack region. This is quite an achievement in itself, considering that most western designs really need computer control to fly at an angle of attack of more than twenty-something degrees. –  Peter Kämpf Jun 19 at 19:56

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