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Why don't airliners have a navigation GPS as "back up" in case of a pitot tube malfunction? Aeropero 603 and Air France 447 plus many other im sure, could have been saved if these aircrafts would have had a simple Navigation GPS as a plan B. The GPS could have given the pilot the basic information needed to keep flying instead of leaving them confused about what the instruments were showing due to the blocked pitot tubes. having reliable information of ground speed, heading and altitude would have helped the pilots recognize the situation they were in, which was caused by Pitot tubes blockage. Most private pilots now use GPS why not the big airliners? There is many blogs about why airliners do not have GPS tracking so the planes could be found after a crash, but none talking about why GPS is not use to help prevent some of those crashes.

Pierre Fortin
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    The pitot tube is not used for ground speed, but for indicated airspeed (and calibrated airspeed when corrected for instrument error, true airspeed when corrected for temperature and pressure). Knowing your groundspeed from a GPS is not going to get you your airspeed, and it's the indicated/calibrated airspeed that is telling you where you are within the flight envelope. The higher you go, the greater the difference between groundspeed and indicated airspeed. At 40,000 feet, you're going to have well over 100 knots difference between ground speed and calibrated airspeed as I remember. – Terry Feb 01 '15 at 01:13
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    @Terry Very true. The difference will be even larger when you're flying in a 180 mph jet stream. On airliners flying through the Northern polar jet stream, I've seen ground speeds that would have been well over Mach 1 in still air. – reirab Feb 01 '15 at 05:32
  • To notice as well that GPS belongs to USA army. If an airplane manufacturer wants to sell airplane to country not alligned to USA, it should take care of not having a single failure point due to not availability of GPS. USA already darked Kosovo. – Trebia Project. Feb 01 '15 at 13:52
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    @Trebia "USA already darked Kosovo." That's interesting. Do you have a link for that? It seems like that would be really hard to do for such a small area (relative to the area that the signal from a satellite would hit,) unless, of course you just jammed the frequencies from the ground. – reirab Feb 01 '15 at 14:48
  • If GPS won't provide airspeed, why not use other methods? – dotancohen Feb 01 '15 at 17:42
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    @reirab in response to pressure from the FAA, in 1996, Bill Clinton signed a bill precluding Selective Availability from being used after May 2000. It seems like Trebia's information is about 15 years out of date. Newer GPS Block III satellites (est. launch 2016) supposedly don't even have the feature. Here are some interesting error plots just before and after SA was terminated. – Nick T Feb 01 '15 at 21:55
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    Hmm, actually we already have this question: Could the GPS be used to aid the autopilot with speed? – hmakholm left over Monica Feb 01 '15 at 22:14
  • @HenningMakholm These look like different questions to me. The other one is related, but this question is "why don't airliners have GPS?" (to which the answer is that they do have GPS.) – reirab Feb 02 '15 at 00:53

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Airliners do generally have GPS receivers, but GPS doesn't give you the information that's needed in this case.

In AF 447, the primary consequence of the blocked pitot tubes was that the flight envelope protection system couldn't do its job properly and therefore automatically disengaged together with the autopilot. The subsequent crash was caused by the pilots' confusion and failure to realize which consequences this had for the aircraft's reaction to their control inputs.

However, what this system needs to function is not something a GPS system can possibly provide. GPS knows how fast the aircraft is moving relative to the earth, but for flight envelope protection you need to know how fast the aircraft is moving relative to the surrounding air, which GPS doesn't have any way of knowing. The differences between these values is caused by wind and can be pretty large. It would be a recipe for disaster to let the computer limit what the pilot can do based on an arbitrary assumption that there's no wind. (And, as Terry comments, there's an additional difference caused by air pressure).

If I remember correctly, the AF 447 investigation concluded that the pitot tubes had thawed and began functioning properly well before impact; it was not the loss of airspeed indication that prevented the pilots from salvaging the situation. By this time the pilots were confused and thought the airspeed indication was still malfunctioning because it showed a much slower speed than they thought they ought to have -- whereas in fact it was working perfectly well.

hmakholm left over Monica
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  • On the other hand, one could theoretically mix in satellite weather and GPS-based altitude to get an idea of indicated airspeed. Though I don't know how accurate that satellite data is for wind speed at different altitudes. – raptortech97 Feb 01 '15 at 04:30
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    @raptortech97: I don't think satellites have the foggiest idea about wind speeds and directions inside a thunderstorm. – hmakholm left over Monica Feb 01 '15 at 04:32
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    @raptortech97 Satellite weather data doesn't directly provide wind information at all, let alone with enough accuracy to use for an aircraft's airspeed indication. You can get some idea of winds by seeing how fast the clouds are moving, but this is only helpful for determining averages over a large area, not for giving an accurate reading for a particular position/altitude combination. If we could get accurate wind information from satellites, Hurricane Hunters wouldn't exist. – reirab Feb 01 '15 at 05:26
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    Wind would cause true airspeed to differ from ground speed, but it is vital to know indicated airspeed, which you can get from true airspeed only by knowing the pressure altitude. Given an altitude from a GPS (if you can trust it) you could estimate pressure altitude, but with an error that depends (again) on current local atmospheric conditions. There may be additional complications I haven't mentioned. This makes for a very complex calculation with lots of opportunity for error. – David K Feb 01 '15 at 13:52
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    @DavidK IIRC, airspeed is one of those nice things where people thought they should be measuring one thing (TAS) that was very hard to measure, but it turns out that the approximation that is easy to measure (IAS) is actually the more important number. – cpast Feb 02 '15 at 00:16
  • @cpast: Angle of attack would be even better. – copper.hat Feb 02 '15 at 01:20
  • @cpast Yes, that's the nice thing about a pitot tube (at least when it isn't iced over). – David K Feb 02 '15 at 02:06
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The short answer to this question is that commercial airliners do have GPS and they have for a long time. AF447 almost certainly would have been equipped with GPS. While what Terry said about the difference between airspeed and ground speed is very true, AF447 could have seen from their GPS unit that they were moving much slower than what they should have been.

Unfortunately, judging from the cockpit voice recorder transcripts, it seems that they were panicking and failing to think logically or communicate well with each other. It was almost entirely a case of pilot error, not lack of accurate information. The pilot in the right seat was almost continually pulling back on his side stick. This caused the aircraft to climb and bleed off airspeed until the wings stalled. Even after the aircraft entered a stall condition, the pilots never lowered the nose, which would have recovered from the stall. Instead, the pilot in the right seat actively held the nose high for the entire descent to the ocean.

Also, the pitot tubes thawed several minutes before the actual crash. All of the instrument indications were correct for quite a long time before the aircraft hit the ocean.

For more information on AF447, see Popular Mechanics' annotated cockpit voice recorder transcript of the incident.

reirab
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Airliners do have GPS units. As the other answers and comments have indicated, GPS does not provide airspeed (or angle of attack) information.

It would seem reasonable to use the GPS as a backup in the following way: Under normal operation, the difference between TAS and the GPS estimate (allowing for some computational latency) should be fairly constant (the difference being the wind speed). When the difference exceeds some noise threshold for a period of time it issues a warning.

This wouldn't help in the case of Aeropero 603 but may have helped in the Air France 447 incident. (To clarify what I mean by help, I mean to reduce cockpit confusion.)

In the former case, a similar tracking system between the radar altimeter and the barometric altimeter may have helped.

copper.hat
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  • It's not clear how you think that would have helped AF 447. The flight computers did realize the airspeed indication was unreliable -- that's why they went into alternate law and disengaged the autopilot. From that instant onward, the pitot tubes were completely irrelevant to the accident sequence. The pilots were not prepared to be given manual control at that point in the flight, and in their confusion ended up flying the fully functional aircraft into a stall so it dropped into the sea. – hmakholm left over Monica Feb 01 '15 at 21:23
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    Also, AF447 was flying through a thunderstorm at the time of the accident. Thunderstorms are characterized by strong turbulence, that is, dramatic local variation in wind speeds and directions, strong updrafts and downdrafts close to each other, etc. So your assumption that the wind velocity would be "fairly constant" wouldn't necessarily hold in those conditions anyway. – hmakholm left over Monica Feb 01 '15 at 21:32
  • In addition to what @HenningMakholm said, pilots use indicated airspeed, not true airspeed. At the altitude airliners fly, the difference between true and indicated airspeed is very large because of the much lower atmospheric pressure at those altitudes. – reirab Feb 01 '15 at 21:49
  • It may have helped to have an unreliable airspeed indication which may have given the crew some idea of what was happening. This might have lessened their confusion. Knowing that the icing incident was over may also have helped. – copper.hat Feb 01 '15 at 21:50
  • Also, this does not actually answer the question that was posed. Answers on StackExchange sites should always answer the question. Please see [answer] for more information on how to write an answer on StackExchange. – reirab Feb 01 '15 at 21:50
  • @copper.hat I'm almost completely certain that the system did indicate both when the airspeed was not correct and also when its proper function was restored, but someone more familiar with flying Airbuses can correct me if I'm wrong here. – reirab Feb 01 '15 at 21:54
  • @copper.hat, repeat after me: The accident was not caused by the pilots lacking airspeed indications. It was not caused by the pilots being unaware that the pitot tubes had failed. The causal role of the failing airspeed indication was completely played out once it had caused the FBW computer to switch into alternate law and disengage the autopilot. From that point on the sole cause of the crash was the pilots' lack of familiarity with how the aircraft handled in cruise conditions under alternate law. Telling them anything about pitot tubes at that point wouldn't have helped the least. – hmakholm left over Monica Feb 01 '15 at 21:54
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    The only thing that it could have saved the flight to tell the pilots was: "The controls are now in alternate law and will let you fly into a stall. Take care not to do that". Telling them that doesn't depend on having a fanciful backup airspeed determination system. – hmakholm left over Monica Feb 01 '15 at 21:56
  • @HenningMakholm: There is no need to so patronising. I never suggested a cause, so I'm not sure why you are ordering me to repeat your words. The pilots do receive an indication that ALT1/2 are in effect. Knowing what triggered the degradation may have helped with their confusion. – copper.hat Feb 01 '15 at 22:04
  • @copper.hat: You suggested a cure, which only has a chance of being effective if one assumes a cause that the suggested cure would influence. – hmakholm left over Monica Feb 01 '15 at 22:07
  • @HenningMakholm: Well, my take on reading the reports was that cockpit confusion was the issue. My suggestion above was a warning which may have helped reduce confusion. (My answer seems to have bothered you, and frankly I don't understand. If you have an issue with me, please write to me at joe.higgins@gmail.com and we can deal with it off this site. This exchange is tiresome.) – copper.hat Feb 01 '15 at 22:18
  • @copper.hat So am I. All I was saying is that 1) This post isn't an answer and 2) TAS doesn't really help you at 37,000 ft. and even less when they went over 40,000, where the flight envelope consists of a very narrow range of Mach numbers. Also, sorry for the canned [answer] comment. I just saw your rep on Aviation and didn't look at your rep on Math, so I thought you were a new user at first. I tend to post comments like that to help out new users. – reirab Feb 01 '15 at 22:22
  • @reirab: No problem at all. – copper.hat Feb 01 '15 at 22:29