Take-off overspeed V2
Author Topic:   Climb gradients
quid
PPRuNe Flight Deck Qualified

Posts: 225
Registered: Jul 1999

posted 06 January 2001 20:55     Click Here to See the Profile for quid   Click Here to Email quid     Edit/Delete Message

max lenz-

V2 speeds, which are driven by Vmca and Vs considerations are not the best "steady state climb gradient" speeds. It is simply a speed chosen to maximize a number of different concerns. Getting off the runway, missing obstacles, noise, etc.

A greater flap setting will get you off the runway quicker, but you sacrifice gradient capability. Once off the runway, you must consider obstacles. A lesser flap setting (with the steeper gradient) is better for distant obstacles, but the greater flap setting will help you miss the closer ones.

For any given flap setting, the published V2 speed sacrifices gradient capability. The best gradient capability is roughly V2 + 20 or 30 knots. The best RATE of climb is even fster.

So if the airline you mention is using 1.4 Vs, they are using better gradient capability than most of us. I'm sure if that is approved, that the obstacle data has been calculated for that takeoff condition.

IP: Logged

mutt
PPRuNe Flight Deck Qualified

Posts: 450
Registered: Sep 1999

posted 07 January 2001 06:05     Click Here to See the Profile for mutt     Edit/Delete Message

Good Morning Quid,

I think that Max Lenz was talking about the Airbus policy of "optimized V-speeds", this is also called "overspeed or improved climb", its basically a case of trading excess field length for speed.

It gets pretty confusing to see a 30 knot V1 increase for a 5,000 kgs increase in weight, but this happens at lower weights on the A300.

Boeing and McDD will allow you to do the same thing, the following blurb comes from some Boeing notes on the subject.

For best runway performance, the aircraft manufacturer will endeavor to establish the lowest possible takeoff speed schedule consistent with FAR25. This philosophy results in V2 speeds which are invariably somewhat below the optimum for climb gradients.

The improved climb procedure is a technique to increase the normal V2 speed towards the optimum gradient speed. The resulting gradient improvement at a given gross weight can then be traded for a higher gross weight at the same gradient.

If the V2 has to be increased then the associated V1 and VR must also be increased in order that the V2 for improved climb can be reached by 35 feet. Obviously the higher speeds for takeoff will require additional field length which must be available in order to use the improved climb technique to any degree. Additionally, at higher speeds, tire speed limits and brake energy V1 restrictions not encountered in the normal takeoff case may come into the problem.

The technique can also be used to enhance obstacle clearance capability.

Mutt

IP: Logged

max lenz
PPRuNe Line Training

Posts: 11
Registered: Aug 2000

posted 19 January 2001 17:30     Click Here to See the Profile for max lenz   Click Here to Email max lenz     Edit/Delete Message

thanks for the answers; what do you think about following idea as a result of 90% of my a320 take-offs beeing done with 1.4 vstall:
now you have the chance to transfer speed energy back to altitude should you get in trouble just after decision "go".
a simulator exercise on a 321 with max take-off weight and engine failure and gear not retractable showed that by "zooming back to vma" we were at about circling altitude of 600 feet giving the chance to stay overhead while dealing with the problem instead of flying away from "safe heaven" giving on the other hand no losses due turns.
rgds max

IP: Logged

O\ZON
PPRuNe Line Training

Posts: 4
Registered: Jan 2001

posted 24 January 2001 17:08     Click Here to See the Profile for O\ZON     Edit/Delete Message

V2 of 1.4 VS is very very VERY high.
Are your charts calculated on a fixed V2/VS of 1.4, are are they calculated on the defaulted optimised V2/VS. Even the optimised V2/VS doesn't go up that high all the time.

O/Z

IP: Logged

max lenz
PPRuNe Line Training

Posts: 11
Registered: Aug 2000

posted 28 January 2001 15:27     Click Here to See the Profile for max lenz   Click Here to Email max lenz     Edit/Delete Message

As far as I remember from the introduction course the aim is to go for 1.4 vs whenever possible for the actual weight - dictated by runwaylength and/or contamination affecting "stop".
Would be nice to hear about other strategies...
max

IP: Logged

O\ZON
PPRuNe Line Training

Posts: 4
Registered: Jan 2001

posted 29 January 2001 09:52     Click Here to See the Profile for O\ZON     Edit/Delete Message

Example:
A320-214, CONF 3

EGAA 17, 18deg C, wind +10

ex A)
Optimum V1/VR
Fixed V2/VS = 1,35
MTOM = 60001 limit code 4-4
speeds 138 146 148

ex B)
optimum V1/VR
optimum V2/VS (in this scenario optimum is 1,13)

MTOM = 67829 kgs limit code 4-4
speeds 134 134 136

to point the obvious, in this scenario the lower speeds produced the best takeoff.


O/Z


IP: Logged

max lenz
PPRuNe Line Training

Posts: 11
Registered: Aug 2000

posted 30 January 2001 14:46     Click Here to See the Profile for max lenz   Click Here to Email max lenz     Edit/Delete Message

Thanks for exemple O\ZON.
So is my understanding of the strategy wrong when I say:
Because the angle with 1.4 vs is steepest it is best for climb limits to be as close as possible to 1.4vs in case of no very close limiting obstacle - where the 1.2 and the 1.4 vs climb outs cross each other.
In your second exemple the reason for the low speeds should be a short runway...
After all, the reason to bring up the subject is that I'm flying a F100 again and unfortunately here the 1.2vs climb out speed is the end of the game.
I would like to know whether only Airbus is using variable v2 speeds or not; who can help and eventually provide an exemple for F100 with the difference in possible take-off weight thanks to higher climb speeds?
Max

IP: Logged

747FOCAL
PPRuNe Flight Deck Qualified

Posts: 23
Registered: Dec 2000

posted 01 February 2001 01:57     Click Here to See the Profile for 747FOCAL     Edit/Delete Message

Quid,

Thanks for the breath of fresh air. Contained in your original post is the key to the qustions I have been asking.

IP: Logged

quid
PPRuNe Flight Deck Qualified

Posts: 225
Registered: Jul 1999

posted 01 February 2001 04:41     Click Here to See the Profile for quid   Click Here to Email quid     Edit/Delete Message

747FoCal-

Thanks. You may be interested in the "V2-reference 35 or 50" thread.

IP: Logged

O\ZON
PPRuNe Line Training

Posts: 4
Registered: Jan 2001

posted 02 February 2001 15:01     Click Here to See the Profile for O\ZON     Edit/Delete Message

max lenz... increased V2 increases climb grad, but it also lesves u on the ground for longer gaining your speed. Hence nearer the obstacle.

i believe the F100 has an optimised V1/Vr option and a limited optimised V2/Vs option. Limited in that it can optiise between 1.2 to 1.3 in increments of 0.02. As far 1.4Vs with the F100, ive never heard of it. But then again, i haven't looked at F100 AFM in quite a while.

As to who can provide you with such analysis, advise asking your Flight Ops Engineer to run these calculations.

O/Z

IP: Logged