Reply to thread 116
quote. I used the CORRECTED math, which is the one that would have put the crew into a stall at Vref-40 kt.
So again we are saying that using GS can be proven unworkable because of something I wrote earlier. I have reworked the argument since then without, I think, mistakes and it the basic premise that GS is a tool that is much misunderstood in its usefulness (or otherwise of course) which should be addressed.
If I were to prove that the world is round in a dissertation and accidentally suggested that the earth is 93,00,000 miles from the sun and has a circumference of 24 000 km, then some using this logic, might suggest that world might be flat. Because I made an error(s). But despite those errors the earth is still round, unless of course one belongs to that society in London.
So to the key points
We all accept now, and we did not at the start, except some still calling GS "bogus" , that GS has its uses and everyone seems to agree that where you have Airspeed Disagree, GS is great for sorting out which of three ASIs is the functioning one. Transfer control to other pilot. Continue drinking tea which didn't even spill. That's the easy one but even that I have seen screwed up into a nightmare if badly handled.
Now, a dark and dirty night with all three ASIs frozen at differing speeds, and perhaps overspeed warnings coupled with underspeed and possibly stick shakers. AP has disconnected. First Officer continued to follow Flight Directors for a while which has messed up the pitch and power. So you are there with almost no valid information and I bet very frightened.
The only source now is a set of tables in the QRH which must be accessed and the correct set extracted from many different configurations - and of course our first officer is not familiar with these tables having only seen then during training. And the tables only work if you at the flight level, weight and configuration stated AND the SPEED stated. if during the upset just related, the speed has gone elsewhere (AF447 and ET being examples where the speed was a couple of hundred knots away from the condition that pre-existed, they are of no use at all.
To the rescue our GS whereby I just fly the GS that I see now, or return it to where it was.
In this situation I would simply set my approx. Pitch Power depending on Flight Level ,which knowledge no pilot should be without, and maintain my safe groundspeed which existed at the time of the failure. Or as stated on the flight plan. Or what I remember it to be. Thats it, safe and a long way from either stall or overspeed.
Taking an easy one. I have this problem after takeoff clean. Maybe FL 50 climbing. Set "six and sixty" and 250 GS. And level off. Don't accelerate or decelerate. Calm restored. Now consult QRH which says for 52 tonnes, 240 knots, 6 deg pitch and 64% N1 (data from 737-400 QRH date 2010 ). Set 65% and monitor GS so that it stays around 250 kts. Can adjust for wind of course. Return to land using pitch and power but keeping the speed in the band quoted in the tables using GS all the way down.
I think that is the briefest explanation I can offer without the A level maths.
And I have done it umpteen times and it works.
Safe flying.
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