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Reduced Thrust on Contaminated Runways

 
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admukhtar



Joined: 26 Jan 2014
Posts: 3
Country: Pakistan

PostPosted: Sun Jan 26, 2014 9:55 am    Post subject: Aircraft Performancep Reply with quote

If you can kindly furnish some answere on why we are not allowed to use assumed temp thrust on contaminated runways where as we can use fixed derate thrust. Thanks.
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K.Haroon
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Joined: 11 Dec 2005
Posts: 1296

PostPosted: Sun Jan 26, 2014 1:14 pm    Post subject: Reply with quote

Takeoff in these conditions require lowering the V1 but you cant lower the V1 below VMCG.

VMCG and VMCA are based on the full rated takeoff thrust.

Assumed Temperature Method (ATM) would give a thrust that is less than the full rated takeoff thrust but would not lower VMCG.

Thats the reason that this thrust is not an operating limit and any time during takeoff thrust levers may be advanced to the full rated takeoff thrust.

On the other hand a fixed derate thrust is an operating limit since VMCG and VMCA are based on the derated takeoff thrust.

Thats the reason that advancing thrust levers beyond this limit can cause directional problems in case of an engine failure.

However the advantage is that a derated takeoff thrust allows a lower VMCG as compmared to an ATM thrust.

When performance is limited by VMCG, such as on a runway contaminated with standing water/slush/snow/ice, a lower VMCG has advantages.
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admukhtar



Joined: 26 Jan 2014
Posts: 3
Country: Pakistan

PostPosted: Sun Jan 26, 2014 3:40 pm    Post subject: Reply with quote

Thanks a lot. One more related question.
Through either method we are reducing the thrust(fixed derate method or assumed temp method). Is there any special tech reason that assumed temp method does not allow lowering of the Vmcg. Secondly, if I have applied fixed derate method in order to take advantage of a lower Vmcg why can't I apply assumed temp on the fixed derate under contaminated conditions.
Thanks.
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K.Haroon
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Joined: 11 Dec 2005
Posts: 1296

PostPosted: Mon Jan 27, 2014 5:42 pm    Post subject: Reply with quote

The objective of reduced thrust through ATM is different as compared to a derated thrust. It takes advantage of the available runway length when full thrust is not required. However the speeds that are applied are with full thrust.

Jet tansport Performance Methods by Boeing would explain this in more detail:

http://www.theairlinepilots.com/forumarchive/quickref/jettransportperformancemethods.pdf

See also Chapter 27: Reduced Thrust for Takeoff

On the contrary, for derate thrust takeoff, minimum control speeds established are based on the derate thrust that is used for takeoff and not less than that. The basic philisophy (if I am not wrong) seems to be the same whether its "ATM thrust reduction on a derate thrust" or two of these separately.

Stopping will be a major concern on a contaminated runway. A reduction of the VMCG (derate thrust takeoff) will allow a reduction of the Accelerate Stop Distance for a given takeoff weight. Thus a better takeoff performance when the MTOW without derate is "Accelerate Stop Distance - VMCG limited". ATM method wont help in this case.

I'll be a bit buys for the next 5-6 days. Once I am free and if the issue is still not clear then let me know, I'll see if I can find some expert on this subject.

Regards
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admukhtar



Joined: 26 Jan 2014
Posts: 3
Country: Pakistan

PostPosted: Tue Jan 28, 2014 10:15 am    Post subject: Performance Reply with quote

I am grateful for your reply. And thanks for the reference book as well. Nevertheless, as you mentioned, if you can get further clarification from some performance expert to corroborate what ever you mentioned would be wonderful.
Thanks a lot once again.
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