VNAV410
Topic Author
Posts: 1
Joined: Wed Feb 08, 2012 5:07 pm

1500 Foot Thrust Reduction

Wed Feb 08, 2012 10:37 pm

Hello all,

This may seem like an obvious Q, but here goes!

I am a 73NG Captain and still have trouble finding out if the thrust that the computer gives you for a particular take-off is the thrust to carry you to 1500 AFE, barring any QRC items. So, if you put in a tailwind you determined to be at 1000 feet, with the surface wind calm, would that be an appropriate wind to input, even though it is not a wind at take-off? long Q! Would doing that mitigate a possible reactive windshear?
Thanks in advance.
 
KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 7:13 am

I would mitigate windshear risk using the following techniques:

If windshear is reported, double the reported gain or or less in knots and delay your departure by that many minutes.
Use maximum thrust instead of a reduced thrust setting for takeoff and climb
Use the highest permissible flap setting for takeoff in your aircraft
Use the longest suitable runway
Use the flight director

Reference your operator's performance calculation methods to see if they have a "suspected windshear" option for takeoff speed and perforamance calculations. This will incrase Vr and V2 up to the performance limits and give you greater stall margin should windshear be encountered after liftoff.

I wouldn't improvise with wind input techniques unless that method was published and sanctioned by your operator.

The thrust settings your computer gives you should take into account all factors including runway condition, length, obstacles, aircraft performance, weather, deferred maintenance items etc. It should be adequate to maintain that setting to 1500 AGL (assuming the NADP-1 departure profile is used).

However, with windshear suspected, I would simply recommend simply using maximum thrust! This guidance has been consistent among the various operators I have flown for.

Hope this helps.
 
SAAFNAV
Posts: 350
Joined: Wed Mar 17, 2010 5:41 pm

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 10:31 am

Wouldn't you use a constant rate climb profile, i.e. the wind at 2/3rds??
Then a 1 000' would be correct.

Erich
On-board Direction Consultant
 
Fabo
Posts: 1150
Joined: Tue Aug 16, 2005 1:30 am

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 5:27 pm

Quoting KAUSpilot (Reply 1):
Use the highest permissible flap setting for takeoff in your aircraft

Wouldnt low flaps give you better climb performance, or is this not what you want?
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
 
Mir
Posts: 19107
Joined: Mon Jan 05, 2004 3:55 am

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 6:37 pm

Quoting VNAV410 (Thread starter):
I am a 73NG Captain and still have trouble finding out if the thrust that the computer gives you for a particular take-off is the thrust to carry you to 1500 AFE, barring any QRC items.

This is entirely dependent on your airline's climb profile. You're going to be using takeoff thrust to somewhere - whether that's 400 feet, 1000 feet, 1500 feet or some other altitude is a matter of company policy.

Quoting VNAV410 (Thread starter):
So, if you put in a tailwind you determined to be at 1000 feet, with the surface wind calm, would that be an appropriate wind to input, even though it is not a wind at take-off?

Depends on the circumstances. If your normal climb profile is to use takeoff thrust all the way to 1500 AGL, then I'd say that inputing the wind at 1000 feet would be a good idea. If you're already going to be at climb thrust when you get to 1000 AGL, then it wouldn't make a difference (though if you really expect windshear, I wouldn't be using anything other than full thrust for takeoff).

Quoting VNAV410 (Thread starter):
Would doing that mitigate a possible reactive windshear?

Again, that's dependent on the speed your airline specifies for your climb profile. Your thrust setting shouldn't have anything to do with whether you get a windshear alert or not.

Quoting Fabo (Reply 3):
Wouldnt low flaps give you better climb performance, or is this not what you want?

I think he meant the setting at which the flaps are least extended. If you can use Flaps 5 instead of 15, do so, as you'll come off the ground at a faster speed (speed is good) and you'll have less drag in the climb (drag is bad).

-Mir
7 billion, one nation, imagination...it's a beautiful day
 
Fabo
Posts: 1150
Joined: Tue Aug 16, 2005 1:30 am

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 6:46 pm

Quoting Mir (Reply 4):
I think he meant the setting at which the flaps are least extended. If you can use Flaps 5 instead of 15, do so, as you'll come off the ground at a faster speed (speed is good) and you'll have less drag in the climb (drag is bad).

That is what I figured, so I asked.
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
 
KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 7:28 pm

Quoting Fabo (Reply 3):
Wouldnt low flaps give you better climb performance, or is this not what you want?

A higher flap setting e.g. flaps 20 instead of flaps 10 is what you want. A higher flap setting like this will give you more stall margin and better performance at low airspeeds, which is the danger during low level windshear (airspeed getting too low and causing a stall or degrading your climb performance to the point where you cannot avoid ground contact).

Quoting Mir (Reply 4):
I think he meant the setting at which the flaps are least extended. If you can use Flaps 5 instead of 15, do so, as you'll come off the ground at a faster speed (speed is good) and you'll have less drag in the climb (drag is bad).


This is what the Boeing flight manual for the 747-400 says anyway.....use flaps 20 instead of flaps 10 for departure into suspected windshear. Other airplanes may be different, but I'd imagine that the 737 is the same. I'm not an engineer, so I don't come up with the reasons why....but this is what the manual says:

 
KAUSpilot
Posts: 1659
Joined: Mon Jan 28, 2002 2:15 pm

RE: 1500 Foot Thrust Reduction

Thu Feb 09, 2012 7:42 pm

Second post to add:

Another reason for using a higher flap setting on takeoff into suspected windshear on airliners is probably tailstrike mitigation. If you were using a minimum flap setting and experienced a sudden loss of airspeed during rotation, there might be a tendency to over-rotate and strike the tail on the runway, resulting in severe damage to the aircraft i.e. MK airlines in YHZ (although that one was simply due to miscalculated performance numbers). A higher flap setting increases tail strike clearance because pitch attitude is lower when the aircraft lifts off. In smaller aircraft this is probably not an issue.

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