smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Fly Into Ground Vs Stall Into The Ground

Tue Oct 09, 2012 7:51 pm

In the QRH checklist for the 737 NG which uses Flap 15 for landing there is a Go-around procedure review.

In it the checklist states that in the event of go-around the pilot should either fly the corrected Vref speed or the minimum manoeuvre speed (top of the amber band) which ever is LOWER.
I cannot seem to understand the reason for using the lower speed of the two. Someone mentioned that it is similar to what a pilot would do like with the BA 777 at EGLL when they lost both engines. The pilot in that case pitched down to gain airspeed. This reduced speed reduced the G and it was better to fly the aircraft into the ground than stall it into the ground.
If anyone could explain the science behind this I would be very grateful.

Thanks
 
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litz
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Joined: Wed Dec 24, 2003 6:01 am

RE: Fly Into Ground Vs Stall Into The Ground

Tue Oct 09, 2012 8:42 pm

I imagine it's a difference between controlling one's descent, and - literally - falling out of the sky.

In the former case, you have control (even if marginally) over your descent rate.

In the latter case, you fall at the force of gravity.

I can't see any instance where the latter isn't going to be greater than the former ... and I would think if the end result is an impact with the ground, you'd want it to be minimized as much as possible.

Stalling has to be the absolutely last thing one would ever want to do, in any circumstance, since it involves a loss of control.
 
Fabo
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RE: Fly Into Ground Vs Stall Into The Ground

Tue Oct 09, 2012 9:05 pm

Litz, I think you misunderstood the question, but for the other part you are right. On the other hand I dont see what the question has to do with the title.

Anyway,

Quoting smartt1982 (Thread starter):
In it the checklist states that in the event of go-around the pilot should either fly the corrected Vref speed or the minimum manoeuvre speed (top of the amber band) which ever is LOWER.
I cannot seem to understand the reason for using the lower speed of the two.

Think this. Thrust is what effectively makes aircraft move. It makes aircraft move forward and upward, granting it potential and kinetic energy.
Effectively, you use X amount of thrust to counter current drag. More speed means more drag, less speed means less drag (remember now we are talking airliners on typical speed, no 2nd regime funnies). You have Y amount of total thrust.
To use Y-X to climb, you want the X to be as low as possible, in effect to be the speed as low as possible.

Now, the method of calculating Vref corr and Vman (or whatever they call it) is such, that it assures that on neither of these speeds you run the risk of stalling. Therefore of these two, you select the lower one, and you can be sure you will not stall. Ideally, you would fly even slower and climb even faster, but that is dangerous since you are not guaranteed sufficient protection from stall.
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mandala499
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Joined: Wed Aug 29, 2001 8:47 pm

RE: Fly Into Ground Vs Stall Into The Ground

Tue Oct 09, 2012 10:33 pm

Lower of Top of Amber band speed or Adjusted Vref?
Top of amber band = minimum maneuver speed for capability... valid at CURRENT configuration.

If adjusted Vref is above top of amber band, your climb won't be optimal.
If adjusted Vref is below top of amber band (but above red-black), you're climbing with a Alpha closer to A-Cl-Max but not at A-Cl-Max (somewhere below red-black top)... BUT, keep your wings level.

The visual picture on the PFD's ASI tape should show you, "don't turn much!"

You also need to consider single engine situations, and also engine failure during go-around...

But, also remember, the amber band slides when you change flap config... the Vref, jumps. Where to pick while in transit? Simple, lower of the two values.

Whatever it is, the absolute limit is the top of the red-black.   
As long as you're above that, yeah, you can pick the lower of 2 choices...   
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