Airspeed and altitude : why not GPS as a backup to pitots ?

[uski]

Hi there,

I'm not a pilot, not even a professional of the industry.
I'm just an engineer with an interest for aviation.

I have been reading a few reports about some crashed such as the AF447 Rio-Paris or the AP603.

Both crashes, it seems, have been related to problems with the lack of reliable airspeed and altitude information from the differential pressure sensors outside the aircraft (pitot tubes).

I have a simple question : why is there no backup GPS system on the plane ?
It would be able to tell the crew its ground speed and altitude in the event the usual system is broken.

I know the crashes might have been prevented by appropriate pilot action, but the confusion over the lack of proper information seems like the first source of the crashes to me.

What's the opinion of you, the pros, about this ?

Let me know !

[Radix]

............

[pattern_is_full]

Question came up in the original AF447 threads.

As briefly as possible:

A plane flies based on how many air molecules are flowing over the wings and controls surfaces per second, generating lift and control forces.

That flow depends on: true airspeed, and air density. Since air density decreases with altitude (up to the vacuum of space), true airspeed must increase with altitude to maintain a given "safe" airflow.

The "magic" of the pitot-tube indicated airspeed system is that it ALSO, in reality, measures air flow (air molecules per second ramming into the pitot tube opening). At sea level, you may be flying 250 knots true speed, and the IAS gauge will read 250 knots. At an altitude where the air is 1/2 as dense, you may be flying 500 knots true, with the IAS gauge still reading 250 knots. Same number of air molecules per second entering the pitot. And since the airflow is what determines performance, the airplane should perform about equally in both cases - at different "true" speeds (engine power is a different problem).

The problem with GPS is that it measures ground speed. True speed (that 500 knots), plus or minus any winds.

OK - suppose you are at 37,000 feet and the pitot airspeed system fails. You glance at the GPS and it indicates "437 knots." Quick - what is your airspeed, and are you safe, or too fast, or too slow?

Winds may be as high as 50 knots (or more) at that altitude, and while you have a forecast, they are notoriously fickle. If you have a tail wind of 50 knots, your true airspeed may be 387 knots. But if it is a headwind, your true airspeed may be 487 knots. How do you know for sure?

Now - quickly - calculate what those speeds equate to in airflow over the wings (i.e what the Indicated Airspeed would be). At that altitude (a simple calculation - can even be a table), and given today's temperature and barometric pressure (which will skew the calculation, and over mid-ocean, may not be accurately known).

Aircraft computers can easily figure true speed based on altitude and indicated speed (and known or assumed atmospheric conditions). And can calculate winds based on true speed vs. ground speed. But only if the pitot system is still working, to provide the "reference" indicated speed in the first place.

[MarkerInbound]

Ground speed is your true airspeed +/- the wind. So depending when and where you are, they may be equal or may be 150 knots off. And if you turn the correction factor will change. Plus pilots use indicated airspeed which is true airspeed after altitude has had its affect on the reading. That being said, most glass airplanes display GS and figure the spot wind based on the difference between GS and air data computer info and ground track and heading.

P I F said it better.

[uski]

Many thanks for your replies. I totally forgot about the wind...

I thought ground speed can "easily" be translated into "airspeed" by looking at the attitude of the plane (accelerometers can do that, to find the angle of the gravity). But I totally forgot the wind.

Case closed I think.

Thanks, it was my first post here, and it's been great :D

[Fly3]

The B787 has backup speed and altitude from GPS.

[pattern_is_full]

Here's a real-life example of why ground speed doesn't help much with flying the plane.

I was in a Cessna 172 on a long cross country flight. Cruising at 100 kts indicated on the gauge (at 11,500 feet, so 126 knots true). Windy spring equinox weather.

I had a radio tuned to a navigation beacon that had DME (distance measuring equipment) ahead of me. Told me how far away the beacon was, and how fast I was approaching (internal electronic calculation based on closure rate). So a fair substitute for "GPS ground speed."

I got a consistent reading of 179 knots! I.E. I had a 53-knot tailwind.

Now - if I had really been going 179 knots, that is 21 knots over the never-exceed speed for a 172. My wings should have ripped off!

If I had lost my pitot IAS indication, and tried to use that ground-speed reading to control the plane, I would have immediately cut power and gently pulled up to get my speed below 158 knots. And slower yet to get back to a "reasonable" cruise speed of 100 knots.

Problem is - my true airspeed would have dropped to 100 knots minus the 53-knot tailwind, or 47 knots. That is the 172's no-flaps STALL speed - and since the winds were turbulent, I'd have likely caught a gust and stalled and spun.

Of course, I'd have gotten a stall warning at a slightly higher speed, leaving me in the "What the heck??!!" situation of conflicting inputs - stall warning, while my one "working" speed indication still said I was doing a safe 100+ knots. Sounds a little like AF447, doesn't it?

[Flo121142]

The B787 has backup speed and altitude from GPS.

So do the newer A320s (and I guess all the other modern Airbus aircraft too)...but on Airbus the speed information is based on the AOA, not on GPS (it displays a green and a red area on the speed tape), but the altitude is GPS...

[mstram]

I'd have likely caught a gust and stalled and spun.

You might / probably would stall in that scenario, but "spun" ?

Pul..lease :cool:..... the 172 is one of, if not THE hardest planes to spin

.. And conversely, the easiest to recover .. let go of the controls and it starts flying again.

[DaveReidUK]

At an altitude where the air is 1/2 as dense, you may be flying 500 knots true, with the IAS gauge still reading 250 knots.So, for a given amount of lift, all other things being equal, TAS will be inversely proportional to density?

Are you sure about that?

[Winnerhofer]

What is Boeing's version of BUSS called?

[Speedbird0390]

Mastram- You might / probably would stall in that scenario, but "spun" ?

Pul..lease ..... the 172 is one of, if not THE hardest planes to spin

.. And conversely, the easiest to recover .. let go of the controls and it starts flying again.




C172s are spinnable, and yes it takes a bit, but are spinnable. During my instructor rating, we were getting the C172 into incipient spins, we didnt let it go past 3 rotations and were able to stop them. All it takes is full aileron at the stall to get them too spin.

[RetiredF4]

Airspeed indicating problems due to icing and the following degradation of airspeed dependent systems had been of short duration, well under 1 minute.

It should not pose any problem to be aware about the wind component of the present flight leg, and what groundspeed that wind resulted in the last minute.
There also should be no problem to record such data and use sose historical data to maintain a safe speed for the short time frame until reliable pitot static information is available again.

But simplest would be to do nothing (in a sense to change as little parameters as possible).

[Denti]

@Fly3: Isn't it AoA based speed indication (computed IAS, not colored bands like the Airbus BUSS) and altitude based on GPS?

[BOAC]

A previous thread for you to read - there have been a few more!

ground speed [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-472591.html)

[stilton]

Amazing how many people think GPS solves all problems :ugh:



AOA is the answer and this is the main component of the backup that Boeing has installed on the B787 with a loss of pitot / static.

[Andy_P]

How exactly would a ground speed help the pilot?

Planes don't fly because of groundspeed.

Did you even read this ?

I'm not a pilot, not even a professional of the industry.
I'm just an engineer with an interest for aviation.

How about instead of being pompous areshole perhaps try and answer the question in a meaningful way. If you cant do that, perhaps dont bother responding it all.

Even noobs like myself as simple questions at times, dont forget that not everyone has the same level of knowledge as qualified pilot.

[Microburst2002]

Actually there is such backup already, but it has important limitations

Air speed is what makes airplanes fly.

Ground speed is what makes airplanes arrive.

Altitude or barometric altitude is something like the reverse of "atmospheric depth". It is where the airplanes fly, it is where airplanes can fly. Atmosphere gets thinner and thinner with increasing barometric altitude.

GPS altitude is just height with respect to the GPS elipsoid.

If airspeed is lost, GPS speed is very useful only to give an idea of acceleration or deceleration. It doesn't give a good idea of speed.

If GPS GS is constant, then airspeed is constant. If the one increases, the other increases too, and so on. GS and IAS are very different at high altitudes, and at low altitudes too, because of wind. It can't be as reference for flight. It gives a limited amount of info, but very useful. If you maintain constant GS in level flight, IAS is constant too.

Same goes for Altitude. GPS altitude can be more than 2,000 ft higher or lower than atmospheric altitude. But, changes in GPS altitude mean that atmospheric altitude is changing. Constant baro altitude means level flight. Thats a very very useful piece of information!

Hope it helps

[Fly3]

Denti.
The first default is AOA speed but if that fails you can have GPS speed.

[Mach E Avelli]

Down in the weeds, at approach speeds, if you did lose all your primary instruments you could use GPS groundspeed and altitude to get it on the ground. Obviously if you know your pitch and power settings, you will be using those as well, but the possible variations of weight, altitude and temperature means they are only 'ballpark'unless you have access to detailed performance data in flight. Older and smaller aircraft are unlikely to have anything other than approximate values.
If you were unfortunate enough to be faced with a limiting runway - e.g. very wet or slippery or minimum length - you'd want to be as precise as possible. Using wind reports from other aircraft and the tower, there is no reason why GPS groundspeed, adjusted for wind component, could not work in your favour.
I have run this as a LOFT simulator exercise, and once they gathered all the information, in almost every case the crew landed quite accurately.
Even in my little bugsmasher I keep an eye on GPS groundspeed and altitude down low just to confirm that the other indications are reasonable.
So, GPS is not totally useless for down low, at low speeds. Better than nothing.

[Speedbird0390]

Most people here (credit goes to the few that have mentioned it already) have forgotten what we were all taught from the very beginning-

POWER + ATTITUDE = PERFORMANCE

[Winnerhofer]

The Boeing 787, from a Pilot's Perspective - View from the Wing - View from the Wing (http://viewfromthewing.boardingarea.com/2012/04/21/the-boeing-787-from-a-pilots-perspective/)
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.

[imriozer]

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...

[Bergerie1]

Speedbird 0390
You are correct!

[Radix]

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[DozyWannabe]

@Winnerhofer:

Same deal with the Airbus BUSS and Backup ALT.

Airbus backs up speed and altitude displays - 3/28/2006 - Flight Global (http://www.flightglobal.com/news/articles/airbus-backs-up-speed-and-altitude-displays-205645/)

http://www.flightglobal.com/assets/getasset.aspx?itemid=12057

Presumably the B787 system also has the same restrictions regarding use.

[Microburst2002]

Speedbird, thank you for enlightening us all.

You inspired us

:D

[Speedbird0390]

Microburst2002, im an instructor, glad i inspired you. That's my job ;)

[Microburst2002]

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

[FE Hoppy]

Let's not dismiss GPS altitude too quickly. After all LPV approaches rely on it. As for GS v dynamic pressure, I concur with the esteemed Instructors who grace us with their contributions ;-)

[Denti]

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.

[Microburst2002]

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

[uski]

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.

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 (http://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007)

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.