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Sunday, May 29. 2011

If it weren't about the AF447 disaster, some comments of readers in some of the forums were actually funny. Especially those ones writing that Air France just had to install GPS in addition to pitots to indicate the speed of the aircraft when the pitot tubes got unreliable due to icing. After all the el-cheapo navigation system for the car shows the speed quite well. Well, while the speed on the GPS display is pretty precise, it has almost no value for keeping the aircraft in the air.

As you you may from school aircrafts are keep in the air by a force called lift. And aircraft has to create at least enough lift force to neutralize the force of gravity. The lift is dependent on the speed you move the airfoil through the air. Okay, that's vastly simplified but sufficient for my explanation. Dependent on the aircraft design the needed speed to create such an amount of lift is different: For example the Fieseler Fi156 has an incredible low minimum speed: 27 knots. Out of this reason this aircraft is capable to fly backwards in really strong winds. But this backwards flying (negative speed if you want to say so) is just in relation to ground … the airspeed is still positive. The difference between the speed of the wind and the speed of the aircraft have just to be larger than the minimal speed of the aircraft and the wind speed must be larger than the aircraft speed. Don't believe me … just go into the next harbor and look at the gulls. Of course you won't see a Boeing or an Airbus backwards … at least not with usual winds.

Another nice example are aircraft carriers: Those have really large engines to accelerate to and to keep top speed. Out of a reason: When an aircraft carrier moves with 30 knots, the air on the flight deck on the aircraft carrier already moves along the airfoil with 30 knots even on calm day. Let's assume a fresh breeze (6 beaufort) or 30kn. When the carrier steers in the direction with the wind, the air would move along the airfoil … well … not at all. When the carrier goes in the direction against the wind, the air would flow along the airfoil with 60 kn.

That's the reason why an aircraft carrier turns into the direction against the wind to start aircrafts. It gives the aircraft a headstart in order to generate lift. And to get an aircraft to 30kn you need really big engines.

To get back to the difference between airspeed and speed over ground? What's the speed over ground? 30 knots. In all cases. But you have three different air speeds. The GPS only measures the speed over ground. However as mentioned above the lift - the force keeping the aircraft in the air - is dependent of the air speed. At 0 airspeed no lift. At 60 kn a lot of lift. At 30 kn airspeed, something in between. And then you have the jet engines and the catapult.

Speed over ground is something different than air speed. And a GPS can't measure air speed. The air speed can only be measured by measuring the speed of the air flowing along the aircraft. To do so, modern aircrafts use so called pitot tubes in conjunction with measuring equipment to measure the static pressure (the ambient air pressure). The pitot tube measures the ram air pressure into the pitot tube. If there is something blocking the air from pushing into the probe, it can't measure the speed.

So … a GPS receiver would have helped the pilots of AF447 not a bit. There are other means like the Backup Speed Scale but that is working completely different.

Posted by Joerg Moellenkamp in Aviation, English at 16:56 | Comments (6) | Trackback (1)

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Hmm, ich stimme da nicht 100% zu. Es ist richtig das ein GPS allein nicht hilft. Aber ausgehend von der Tatsache das ich überhaupt keine Geschwindigkeitsanzeige habe ist es besser als gar nichts. Zumal laut METEO Infos zu AF447 nur ein Headwind von 5-10 knots vorhanden war. In der Tat mag das GPS zur zuverlässigen Anzeige der stall speed irrelevant sein - muß es auch nicht. Aber man kann es benutzen um sich im sicheren mittleren Bereich zwischen Stall und overspeed zu bewegen. Und das ist sinnvoll wenn ich aus dem Stall mich herausmanövriere und beim Sinken zu viel Fahrt aufnehme. Von daher könnte man ein GPS schon sinnvoll als fallback nutzen. Und in Verbindung mit anderen Daten kann ich durchaus noch die ein oder andere Information herausholen...
Und nein, dein backup speed scale funktioniert auch nicht wenn die Pitot Rohre vereist sind. Somit ist GPS kein Allheilmittel aber besser als gar nichts.

#1 Chris on 2011-05-29 21:42 (Reply)

Ich glaube nicht, das man mit der GPS-Geschwindigkeit etwas anfangen kann. Ich muesste ja staendig die aktuelle Windgeschwindigkeit haben, um daraus die Airspeed zu berechnen. 10-5 kt sind ja nur ein durchschnittswert und ich glaube auch nicht, das diese Geschwindkeiten in der nähe eines Gewitters noch Gültigkeit besitzen.

Und gerade in grosser höhe ist die grenze zwischen overspeed und stall speed doch so gering, das man zu schnell darueber bzw darunter ist. So lese ich zumindestens die Dokumente, die ich zur Verfügung habe.

Ausserdem war die Geschwindigkeitsanzeige bei AF447 wohl nur für eine halbe bis eine Minute inkonsistent ... der Absturz dauerte vier Minuten. Das macht diesen Abstutz noch seltsamer....

Zur Backup Speed Scale: Soweit ich weiss basiert die BUSS auf einem getrennten Angle of Attack sensor, ist also von den Pitots unabhängig. Okay, wenn sowohl pitots zufrieren als auch die AoA-Sensoren völlig festfrieren, ist man auch so im Eimer.

#1.1 Joerg M. (Homepage) on 2011-05-29 22:03 (Reply)

Naja, wenn ich die Berichte derzeit richtig interpretiere war das Problem ja auch nicht wirklich die Speed indication sondern die Art und Weise wie sie versucht haben aus dem Stall heraus zu kommen. Airbus und Boeing haben die Recovery Anweisungen hierzu nach dem Unfall angepasst. Dies war wohl aber nicht in Verbindung zu dem Absturz zu sehen sondern langfristiger in Arbeit. Ich bin jedenfalls sehr auf den Absturzbericht gespannt!
Und soweit ich gelesen habe hat Air France ihre Maschinen nicht mit BUSS ausgestattet, nachdem Testflüge nicht so zufriedenstellend waren. Auch empfiehlt Airbus, BUSS nicht oberhalb von 25000ft einzusetzen da es unzuverlässig ist. Und ich muß vorher meine Air Data Reference units permanent deaktiveren um es zu nutzen. Für einen Weiterflug absolut ungeeignet und nur für den absoluten Notfall - wenn man die Gefährlichkeit erkannt hat.

#1.1.1 Chris on 2011-05-29 22:15 (Reply)

I'm just curious and I know nothing about aircraft design but how would a GPS know that its object is not "flying" 20cm from the ground but 5km? And if it doesn't know that, does it mean the GPS metering would be unreliable at that speed due to the Earth circumference? If unreliable is the answer, can it be made reliable?

#2 Giovanni (Homepage) on 2011-05-30 04:11 (Reply)

The speed was not the problem of the pilot. Since the airbus had the nose high up in the air, an additional speed reading wouldn't have changed anything.

The question ist, why did the pilot not get back to normal flight attitude. An airbus is not a fighter jet, which can rise at 30 degrees and more. Was it a shock or a malfunction ? The data available to the public reads more like a shock. It's not easy to turn the nose down, if you are falling. But you need to do it, to get lift again.

As a professional pilot, the pilot certainly knew this. What kept him from getting the nose down ?

#3 Knut (Homepage) on 2011-05-30 11:46 (Reply)

The sidestick of the PF had an up input all the time according to the BEA note. It looks really like a shock reaction. I'm "looking forward" to read the official report at the end ...

#3.1 Joerg M. (Homepage) on 2011-05-30 11:49 (Reply)

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