Airspeed and altitude : why not GPS as a backup to pitots ?
Joined: Oct 2013
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From: Wengen
B788
The Boeing 787, from a Pilot's Perspective - View from the Wing - View from the Wing
If you lose all 3 pitot/static systems or air data computers, the airplane reverts to AOA speed (converts AOA to IAS), and this is displayed on the normal PFDs airspeed indicator tapes.
GPS altitude is substituted for air data altitude and displayed on the PFD altimeter tapes. Very convenient.
If you lose all 3 pitot/static systems or air data computers, the airplane reverts to AOA speed (converts AOA to IAS), and this is displayed on the normal PFDs airspeed indicator tapes.
GPS altitude is substituted for air data altitude and displayed on the PFD altimeter tapes. Very convenient.
Joined: Jul 2012
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From: Around FL380
GPS inputs are great
While in cruise you are supposed to have an estimate wind direction and speed, if your pitot static system freezes in cruise or even during climb/decent the GPS ground speed will do the job. If in cruise at lets say FL370 while doing 450 KTAS you lose your airspeed/altimeter you know approximately what should be the wind and you can calculate what is approximate KTAS, if the GPS is telling you your GS is 200 knots while in cruise and at the last check point the wind wasn't 250kts headwind then you know you are too slow and I was exaggerating...
As for GPS altitude, great indication for your altitude-+ 1000ft
You can even shoot an approach just by using the GS, you know the wind and do the correction for it. Maybe you'll mistake few knots but you'll land...
As for GPS altitude, great indication for your altitude-+ 1000ft
You can even shoot an approach just by using the GS, you know the wind and do the correction for it. Maybe you'll mistake few knots but you'll land...
Last edited by imriozer; 3rd August 2014 at 18:46. Reason: Correction
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From: UK
@Winnerhofer:
Same deal with the Airbus BUSS and Backup ALT.
Airbus backs up speed and altitude displays - 3/28/2006 - Flight Global
Presumably the B787 system also has the same restrictions regarding use.
Same deal with the Airbus BUSS and Backup ALT.
Airbus backs up speed and altitude displays - 3/28/2006 - Flight Global
Presumably the B787 system also has the same restrictions regarding use.
Joined: Mar 2005
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From: Uh... Where was I?
So am I, Speedbird, and probably many of the people posting around here, who where answering the initial question of the thread, and all of whom are well aware of the importance of pitch and thrust and don't deserve your
Most people here (credit goes to the few that have mentioned it already) have forgotten what we were all taught from the very beginning
Joined: Mar 2001
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From: I wouldn't know.
Isn't LPV only available with with WAAS? Which is of course available, but pure GPS probably isn't accurate enough, isn't it?
To be honest, haven't flown any LPV approaches, we are not certified for that, just GLS CAT I approaches (and trial autolandings). But GLS needs a ground segment and is far from being pure GPS.
To be honest, haven't flown any LPV approaches, we are not certified for that, just GLS CAT I approaches (and trial autolandings). But GLS needs a ground segment and is far from being pure GPS.
Joined: Mar 2005
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From: Uh... Where was I?
GPS altitude is very useful because you can level off for trouble shooting with reference to it, specially when you don't rely on your baro altitude indications (not even the FPV).
The technique to fly the UNRELIABLE SPEED procedure is to maintain flight path (level flight) and then see what happens with pitch in order to adjust the thrust required. So you need too know if you are maintaining level flight (if you don't, the whole thing is of little use) and GPS altitude gives you that.
Also, at lower altitudes below transition level it will give you an approximate altitude, but it is advisory only, so to speak.
Groundspeed is no so useful, but at least you can see if you are accelerating or decelerating. of course angle of attack is a much better information to have in the left side of the PFD
The technique to fly the UNRELIABLE SPEED procedure is to maintain flight path (level flight) and then see what happens with pitch in order to adjust the thrust required. So you need too know if you are maintaining level flight (if you don't, the whole thing is of little use) and GPS altitude gives you that.
Also, at lower altitudes below transition level it will give you an approximate altitude, but it is advisory only, so to speak.
Groundspeed is no so useful, but at least you can see if you are accelerating or decelerating. of course angle of attack is a much better information to have in the left side of the PFD
Thread Starter
Joined: Aug 2014
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From: Canada
Thanks everybody for this interesting discussion ! Glad I started it with my noob question 
As to the GPS solving problems, I just want to mention the Korean Air Lines flight 007, which was shot down by a Soviet fighter jet because it inadvertently flew into the Soviet air space. It's one of the event which led the USA to release the GPS for use in civilian applications, to prevent similar errors in the future.
I now understand that ground speed and absolute altitude is not very relevant to an aircraft. In the case of Aeroperu 603, the lack of another altitude source likely was a cause of the crash (flight crew ignored the GPWS because of too many alarms and because they thought they were high enough). This is why I started this thread.
Thanks guys, it was very interesting. And I'm sure many people who are not aviation specialist will likely find this thread and learn from it as I did.
As to the GPS solving problems, I just want to mention the Korean Air Lines flight 007, which was shot down by a Soviet fighter jet because it inadvertently flew into the Soviet air space. It's one of the event which led the USA to release the GPS for use in civilian applications, to prevent similar errors in the future.
President Ronald Reagan announced on September 16, 1983, that the Global Positioning System (GPS) would be made available for civilian use, free of charge, once completed in order to avert similar navigational errors in future.
-- Korean Air Lines Flight 007 - Wikipedia, the free encyclopedia
-- Korean Air Lines Flight 007 - Wikipedia, the free encyclopedia
Thanks guys, it was very interesting. And I'm sure many people who are not aviation specialist will likely find this thread and learn from it as I did.
