allowed error in V2 during OEI climb?

[rhovsquared]

With todays massively over powered twins (737, 757,777 A320, etc.) during EFATO is maintaining V2 as important as it formerly was or does this apply to solely to WAT limited or obstacle limited departures? or does the AFDS command a speed between V2 and V2+X that gives the best OEI gradient for the net flight path, with V2 as the absolute lower limit?

Thanks in advance RHOV

[AerocatS2A]

I think you'll find that the aircraft were certified based on the performance obtained on one engine by flying at V2. Therefore, regardless of how well the aircraft may perform on one at speeds higher than V2, the pilot (or automatics) is/are obliged to maintain V2 as well as they can.

[john_tullamarine]

Some thoughts ..

(a) twins are comparatively over powered to allow for the OEI case .. thrust is down by half but the resulting performance is down by something in the region of 80 percent

(b) for a near-V1 failure, V2 is important as a balance between distance and climb gradient when compared to what is scheduled in the AFM. If you try to climb much below V2, performance is down and other margins (Vs, Vmca, for instance) are reduced. If you try to accelerate to V2 + delta, you spend distance which might compromise runway numbers and obstacle clearance

(c) on the other hand, if the failure occurs shortly after takeoff, the AEO acceleration has put you at V2 + delta and you are better off hanging onto some of that delta in accordance with OEM guidance (typically V2 + 10-20)

(d) if you are WAT-limited, you don't have much any sort of spare performance and the available speed range will be fairly narrow ... (b) and (c) still apply

(e) obstacle scenario depends on location and balancing distance gone at failure against actual speed against required gradient .. all in all one does the sums beforehand and then sticks to the script

(f) what the box commands will depend on the certification and whatever programming is embodied

[Old Smokey]

In the interests of compromise between Field Length and 1st/2nd Segment Obstacle clearance climb, V2 is often minimised, primarily a Field length consideration. V2 min is most often somewhat below the best gradient speed available, but necessary due to field length constraints. As John_Tullamarine has indicated, allowing the speed to climb above V2 will consume climb gradient, thus providing less than satisfactory obstacle clearance.

Most (good) AFMs advise a maximum speed to be flown in the event of engine failure above V2 (V2+10, V2+15 etc). This recognises that an improved gradient is possible at these speeds, PROVIDED THAT the engine failure occurs after V2 has been passed. Accelerating to these improved gradient speeds WILL reduce your gradient during the acceleration, perhaps disastrously.

This gives you 3 options -

(1) If failure occurs between Vef and V2, follow the AFM pitch attitude recommendations for acceleration to V2 and then hold V2. This is allowed for in the airborne portion of the Takeoff between Vef and reaching V2 at the Screen Height,

(2) If failure occurs between V2 and the AFM recommended additive (e.g. V2+7 where the AFM has stated 'Not above V2+10'), fly the EXISTING speed without deceleration or acceleration,

(3) If failure occurs above the AFM recommended V2+X, reduce to V2+X, and maintain that speed.

That's how they're certified.

rhovsquared, 737s, 757s, 777s, A320s, etc are NOT over-powered. Sure, on many occasions, there's performance to spare, and we use Flex / Reduced Thrust, but I've done numerous Takeoffs where we NEEDED to use every pound of thrust available.

[rhovsquared]

I greatly appreciate these replies from the a good deal of mentally painful confusion has been cleared up for me, as far as 'massively overpowed' -Sorry I was enthusiastically/childishly thinking of non-limiting cases and the cool way that they depart airports I've lived both by Laguardia and JFK:D
Many thanks RHOV

[john_tullamarine]

.. hey, there's nothing quite as much fun as a positioning flight on a twin at min weight ... not quite in the F15 league but still good clean fun.

[Old Smokey]

Not quite in the F15 league but, had the pleasure of a B777 ferry recently, not even a cabin crew, only the F/O and myself and about 20% of fuel capacity on board.

The F/O and I mutually agreed that the aircraft was long overdue for a check of it's fully rated thrust, so.................

I seem to recall V1 being called just after passing the near end 1500' touch-down marker, and making the right turn to on-course whilst at 1000 feet and only half way down the runway. Good legal fun!:ok:

Regards,

Old Smokey

[john_tullamarine]

.. bit like Trader Jim's homemade 32 Mt Barrow departures out of Launy at oh-dark-30 on the Goose years ago ... may the old rascal rest in peace ...

[fogatgatwick]

extra speed above V2 also increases turn radius, which has the potential to naff-up any emergency turn too..........

[Old Smokey]

A good point fogatgatwick, OEI 'Special' Procedures are designed with the inner radius for the splay at the lowest V2, and the outer splay turn radius at the highest V2+the AFM approved exceedance. With turn radii being a function of speed squared, even a little extra can take you out of the surveyed area into 'No-Man's Land'.

Dammit, I've got to get a life and finish my leave...........................

Regards,

Old Smokey

[JW411]

The other subject which deserves discussion is whether to make a level acceleration or not where a relatively close-in obstacle is concerned.

For example, the emergency procedure off Geneva 23 requires a turn at 6.2D back to St Prex (which is more than a 180º turn).

Making a level acceleration (which we normally do) versus maintaining configuration and V2 - V2+10 to 6.2D and then making the turn at 15º of bank is not such a good idea. The traditional clean up method leaves you 500 feet lower in this case on my current type but nowhere does it tell you this in our (Swedish) RTOWs. (I have proved this in the sim).

I am told that the performance engineers have to make such an assumption when preparing RTOWs for runways with close-in obstacles.

Wouldn't it be nice if the theoriticians were to tell those of us who might have to put their wonderful ideas worked out in a nice warm office into practice on a dark and stormy night?

[john_tullamarine]

Important point in the last three posts .. missing the hard bits depends on planning and executing the departure in 3D .. ie vertical (climb gradient and speed control) and laterally (straight tracking and turn radius which is critically dependent on speed control).


whether to make a level acceleration or not where a relatively close-in obstacle is concerned

Company SOP or special procedure for the runway must specify what the ops engineers expect the pilot to do .. if they don't then you need to go thump the ops engineer's or chief pilot's table ... hard and loud.

Making a level acceleration ... versus maintaining configuration and V2 - V2+10 ... and then making the turn at 15º of bank is not such a good idea

The two cases will give wildly different splays .. if you pick the wrong option that might not be a successful life strategy. While either case can be calculated, the preferred option (if there are real obstacles) is to keep the initial climb speed .. reason ... it all gets too variable in the real world considering the actual gradient profile and speed history to the turning point. Hence, with the acceleration option, the ops engineer should be factoring a lot more fudge and padding to allow for variation from the plan .. end result is usually going to be less payload than the simpler maintain initial climb speed plan. (Caveat .. the early acceleration may be the better plan if the turn is to avoid a simple obstacle and the main limiting critical obstacle(s) is(are) a long way down track. However, the better procedural option most of the time is the low speed case)

I am told that the performance engineers have to make such an assumption when preparing RTOWs for runways with close-in obstacles

As ops engineers (performance engineers .. whatever tag you prefer) we make lots of assumptions .. but an assumption is useless if the guy implementing the story (the pilot) is kept in the dark as to the assumptions .. bit like pin the tail on the donkey.

Wouldn't it be nice if the theoriticians were to tell those of us who might have to put their wonderful ideas worked out in a nice warm office into practice on a dark and stormy night?

Yes, indeed ... not only nice but essential if you want to have a reasonable chance of making pensionable age ... and one of the reasons why the ops engineer who flies (or keeps a close liaison with the flying folk) has a generally better output than the backroom guy who tries to do it in isolation ...

[OzExpat]

Oh for Heavens sake OS! Get away from EVERY bloody computer and enjoy your holiday! :D

I'd be really surprised if the assumptions used by the performance engineers did not translate into printed form for the benefit of the pilot. Normal instrument flight procedures have rules associated with them - and special notes when any of the normal rules can't be applied. And that's before we even think about ATC requirements for flow control and noise abatement.

All of this is spelled out in each procedure whenever some sort of limitation is necessary. I'd have thought that most (if not all) performance engineers would be as aware of the reasons for this as instrument flight procedure designers. Of course, if there's too many notes relating to limitations then it's probably time to re-evaluate the whole procedure...:eek:

[mutt]

The other subject which deserves discussion is whether to make a level acceleration or not where a relatively close-in obstacle is concerned.

Do your RTOW charts show a Mininum Flap Retraction Height (Altitude)? The purpose of this height is to allow you to commence the accleration portion after clearing all obstacles.

Its also worth considering that in GVA you must conduct a 180° turn, if you acclerate in the straight out portion, does your RTOW chart state the maximum speed for conducting this turn?

Mutt

[Capt Chambo]

JW411 and Mutt....

Re:- Geneva runway 23. A common destination for us, we use Jeppesen charts and the Boeing BLT, and these details refer to the BBJ.

From memory, the emergency procedure is to climb straight ahead and at 4DME GVA commence a left turn on to a HDG of 110 (15 degree bank angle)and then when we cross the 195 Radial GVA turn to SPR and take up the hold climbing FL070.

Which if nothing else serve to illustrate that there is more than one way of "skinning the cat" in terms of solving performance problems.