Originally Posted by
threemiles
There is a THIRD and independant IAS speed tape on the flight deck, right above the gear announciators.
GPS delivers ground speed and is of little value, unless you are close to MSL in calm air.
Hi Three Miles. I don't know if you have tried the following but it works ..........
The Standby instruments are basic non corrected airspeed altitude, Horizon and ILS. You an fly a successful approach using just these with all else failed.
An additional value of these third readouts is when you get an disagree between the to main ASI or ALT readouts, the third man can be judge and jury if used carefully.
Indeed that is how you often resolve AIS disagree issues. Which two agree?
Now to Groundspeed GPS. It is a remarkably useful and I would say essential tool in cases of confusion as arose on AF477 and might have been factors in the recent MAX cases. And in many cases total hull loss with loss of airspeed over the years GPS would have got them down safely. You can easily compensate for wind and True Airspeed using simple knowledge of groundspeed at various altitudes.
From cruise altitude right down to touchdown your flight plan has the groundspeed for every leg based on your track, altitude and forecast wind,accurate to within maybe 5 kts. You just fly it using the tables provided by Boeing in the QRH which give pitch and power for every flight condition, weight related. Remember that to crash you have to be wildly out on airspeed. if AF 477 had maintained current GPS speed at time of failure (450 KTS or whatever it was) and not made adjustments to power and pitch, and not got below say 400 kts groundspeed /GPS they could have flown home to Paris, or certainly long enough to work our what was wrong. I have seen demonstrated over the years pilots practice GPS only flying from Altitude to touchdown (on simulators) with no effort whatsoever with NO other data. On 737s during initial type rating training we would put beer mats with blue tack over all the instruments except SBY and GPS and they could fly a perfect circuit even with no ALT readout using GPS and radio altimeter below 2500 feet.
The Tristar taught flying GPS G SPEED approaches in serious headwind landings - none of this is new, but it is being forgotten. So here we have big jets using this technique when it is the very opposite of still air!
Cheers
Y