screen height - 航空论坛_航空翻译_民航英语翻译_飞行翻译

jetset pete

26th Nov 2000, 21:18

well and truly rich now thanks. But not quite enough to pay sponsorship, yet!
So how does the aircraft fly inbetween the two speeds? I thought V2 = minimum flying speed with one engine down. Can V2 be reached before V1 and before rotation on a really long runway?
Yours, puzzled.

[This message has been edited by jetset pete (edited 26 November 2000).]

F/O Junior

26th Nov 2000, 22:14

Between V1 and V2 you have Vr (rotate).

V1 is calculated in such a way, that you are still able to accelerate you aircraft with only one engine on the remaining runway if you have your engine failure after v1. You then accelerate towards Vr where you get up in the air and then further towards V2 (Your minimum safe engine out speed).
On the other hand, V1 ensures you enough runway to stop your bird should the failure occur before V1.
V1 depends on your take off weight, runway length, wind component, QNH, temperature, runway conditions (dry, wet, standing water etc.) and systems that influence T/O performance, e.g. anti ice equipment, which uses bleed air from your engines --> loss in thrust, disabled spoilers or reverser --> loss in braking efficiency.

aztruck

27th Nov 2000, 03:50

To make matters more fun there is the wonderful subject of "screen height". This is an imaginary screen which the lowest bit i.e. the landing gear, has to clear at V2. For an all engines take off screen height is 35 feet. In the event of an engine failure you are allowed to strain and pucker to a height of 15 feet, thus lurching across the M25 safely.
There are all kinds of scams available to use available runway to increase takeoff weight and therefore stagger into the sky at the last possible moment with the max allowable payload.
if t'were not so then international aviation would hang up it's clogs.
If you have ever run into the expression "backside of the drag curve", then you have an idea of the problems facing an airliner at v2.
The fuel laden monster is dragged into the sky at the minimum airspeed by dint of flaps/slats/slots which increase lift but also increase drag.
Once you clean up the wing then the aircraft has a chance to accelerate to a speed where lift will increase with airspeed and induced drag disappears down the plug hole leaving you with excess thrust to shove you away from the awful earth beneath.

mutt

27th Nov 2000, 08:05

Aztruck,

Where did you get your screen height requirement from?

AFAIK, there is no screen height requirement for an all engine takeoff, try finding information for all engine takeoff's in your AFM or Ops Manual. You have to go to the more obscure community noise document.

The 35ft screen height is for cases where you have an engine failure at VEF on a dry runway. The 15 ft screen height can be used for cases where you have an engine failure on a wet/contaminated runway as can reverse thrust.

What other scams can you tell us about?

Thanks

Mutt

Slasher

27th Nov 2000, 12:40

Yeh I second Mutts comment. 35' for failure. No screen height all engines. 15' for a V1 minus 5kt wet (737), or a curved departure requirement at runway head for terrain clearance engine-out (15' obst clearance).

PS Mutt, if I recall right a curved engine-out is a 1% nett minimum legal requirement (with 15 deg AOB max) for a twin in the 2nd segment configuration?

mutt

27th Nov 2000, 14:24

Slasher,

I understand that the NET gradient still stays at 1.6% (Gross Gradient for a 2 engine aircraft 2.4% - 0.8% = NET Gradient of 1.6%.) You still have to achieve this Net Gradient in a bank turn. The turn will decrease your gradient (B737 Flaps 05) by 0.5% which you must account for. We normally do this by increasing the obstacle height or the Net Gradient of 1.6%.

I haven’t found any reference to a minimum of 1.0% NET.

Hope that this helps.

Mutt

aztruck

1st Dec 2000, 03:30

Fair point. My screen height info comes out of the Perf A cap, which is a CAA airliner.
Real ops/real world/real airliner different regs.
Nightmare exam though.

quid

1st Dec 2000, 08:16

jetset pete-

V2 isn't minimum flying speed, it's at least 20% faster than stall. It's just the speed used to meet the takeoff and obstacle clearance requirements.

Royan

2nd Dec 2000, 02:26

V1 : The highest speed , during takeoff , at which the flight crew has a choice between continuing the t/o or stopping the aircraft .
VR : The speed at which the pilot rotates in order to reach V2 at an altitude of 35' at the latest after an eng failure.
V2 : Takeoff safety speed that the aircraft attains at the latest at an altitude of 35' with one eng failed and maintains during the second segment of the t/o.

[This message has been edited by Royan (edited 01 December 2000).]

mutt

2nd Dec 2000, 07:55

Royan,

Your definition of V1 is correct, but you will find that it has developed from a "decision" speed to an "action" speed.

Mutt

Royan

2nd Dec 2000, 08:37

Mutt ,
I got the latest definition from the JAR's . You'll like this one better.thank you ,here it is.
'V1' means the maximum speed in the take-off at which the pilot must take the first action (e.g. apply brakes, reduce thrust, deploy speed brakes) to stop the aeroplane within the accelerate-stop distance, V1 also means the minimum speed in the take-off, following a failure of the critical engine at VEF, at which the pilot can continue the take-off and achieve the required height above the take-off surface within the take-off distance.(Amended by Orange Paper Amendment 1/99/1)

IMMELMAN

3rd Dec 2000, 04:32

Non-technically, we are all supposed to have a "GO" mentality in these situations - ie - V1 Cut - these days, but next few take-offs,when PNF calls V1 - take a real look ahead and ask yourself, quickly, if you think you could really stop at that point
I have to say that, invariably, for me, the answer has been NO!
So, at a later stage, I have recalculated the take-off, come out with all the original speeds, but I had still decided that I would not have been able to stop in any comfort, in one piece, or still on some firm surface - worrying, innit?

Antman

5th Dec 2000, 10:40

Jet Set, you can have a situation where your V2 is less than V1 but only on very long runways. This is known as improved climb profile and we use it at hot & high airports. This effectively does away with the acceleration segment from Vr to V2 as V2 is reached rolling down the runway and enable's you to pick up more load sometimes upto 3000kg on a B737 at a place like JNB.
Elevation 5500' temp in summer between 24'c and 30'c sometimes hotter.

seadog

5th Dec 2000, 13:20

What a lot of waffle. To answer the original question,as far as the CAA are concerned in CAP 385,for Perf A aircraft, if an engine failure occurs after V1 then take off must be made and Vr will be made before the end of the runway, after which acceleration to V2 and the screen height will be made by the end of the clearway.The actual definiton of V2 is "The target speed to be attained at the screen height with one engine inoperative, and is used to the point where acceleration to flap retraction speed is initiated".
Obviously Company procedures may have a slightly different interpretation, but that's it from the horse's mouth.