Landing speeds over Vmbe

[hawk37]

Let's say you've decided in an emergency to land your heavy jet, zero wind, where Vref will be well above your maximum brake energy speed. Is their an AFM procedure to follow? Aero brake until Vmbe, then brakes? Use rev thrust until Vmbe, then brakes? Use brakes first, then keep thrust reversers in use until stopped? What does Airbus/Boeing/SOPs say to do?

[Intruder]

It depends on the runway available... Specific procedures will vary with the airplane, but a general range may be:

If you're at Edwards AFB with effectively unlimited runway, use max reverse and no brakes until below 100 Kt or so.

If you have 12,000', figure out the landing distances for available autobrake settings, and use one that will stop you in 12,000'. Use max reverse as well, so you are not using the brakes alone to stop.

If you have less than that (i.e., not enough runway to stop "by the book"), use max reverse and max brakes, and get as slow as possible as soon as possible.

AFAIK, Boeing does NOT recommend aerodynamic braking in any foreseeable situation. Spoilers and brakes are less effective with the nose off the ground, and control is better with the nosewheel on the ground.

[gearpins]

E=1/2mv^2

Brake energy limits come into play during rejects and most a/c are designed to be able to t/o at max str limit wt and to reject (which is cerified without the use of rev any way) until complete stop using only brakes.(spoilers are allowed ofcourse).consequently in a situation as envisaged above the a/c is defenitely below max to wt. the energy that needs to be dissipated is shared by the revs additionally you normaly would have a longer dist to do it than in a reject.hope that helps

[Mad (Flt) Scientist]

Except that you only have to consider rejecting from max V1 (which may even be limited by Vmbe considerations) whereas the scenario suggested could have a higher touchdown speed (say flaps failed up, high airports only available).

[Intruder]

The scenario that most often comes up in training is a cargo fire just after takeoff. As a recent UPS fire illustrated, you may have NO time to get the airplane on the ground.

In the case of a 747 at max gross takeoff weight, it's a 90/270 and land opposite the T/O runway, unless the tailwind is too adverse. Then, a VERY short racetrack to final on the T/O runway.

With a -400 at LAX on one of the 25s, it's a likely good outcome. With a -200, the old steel brakes mean likely blown tires earlier, and more likely to run off the end.

Note that the max V1 reject has less runway to use than the max gross weight landing. However, the flaps won't come all the way down at approach speed (only to 25 due to automatic blowback), so that's yet another "gotcha"...

[mustafagander]

To contemplate landing above this limitation, there must be a very pressing need. Hence I would expect a pilot to just land and try to make the best of it. I would not try to be cute on approach - not below Vref +5 or additive required, stick to the customary 3* slope and put it on at the aiming point. After that, use all retarding devices available and see what happens at the far end of the runway. The usual maxim applies - it's better to go off the end at 50 Kts rather than fall out of the sky at 200 Kts.

Those B767 drivers here can imagine the scenario of a 185 T departure with flap asymmetry between 5 and 1 on retraction. After the additives the Vref can be rather impressive and few B767s can dump fuel.

[Alex Whittingham]

On the Handley Page Victor there was no reverse thrust, only a brake chute. If you didn't want to stream the chute the procedure was to land and wait and wait as the aeroplane very slowly reduced speed to VMBE, then brake as normal. The Vulcan used aerodynamic braking. On a modern aircraft I would suggest full reverse and continuous braking from VMBE down. In the cargo fire scenario would you have time to calculate VMBE?

[rhovsquared]

Vmbe is usually NOT a ldg factor [except in rare case such as a 757 coming in at MTOW as a result of an emergency] to a low Density alt . I think the factor on ldg is 1.5% of the BE required for stopping at the highest Vref., and NO margins on the RTO at the Highest BF V1.
But as John Tullamarine warns all the time...Check you airplane's certifcation against the Applicable, CARs,JARs,FARs.



To The Tire Speed Limits And Beyond
rhov

[gearpins]

a flt ctrl problem will provide sufficient time to burn off and get the wt under control= for a smoke/fire/medical emer etc the land flaps(in most cases are available)= hence a combination of low/no flaps+over wt landing is a very remote possiblity assume rtow is vmbe limited and you take off(using max v1) followed by an immediate return,remember v1 cannot be greater than vr which inturn cannt be greater than v2, which by the way is about 20% above vs. on the return vref for say the same wt is about 30% above vs. assume a stall speed of 180 kts so we are talking about absorbing the energy equvalent of 18 kts!!.with the luxury of reversers and longer r/w I dont see a problem

[rhovsquared]

Gearpins I was thinking about a very serious 'need to get down now!!!' type of thing...My point was one probably never will approach that Vmbe




Happy S-turns Along The Road
rhov

[Mad (Flt) Scientist]

assume rtow is vmbe limited and you take off(using max v1) followed by an immediate return,remember v1 cannot be greater than vr which inturn cannt be greater than v2, which by the way is about 20% above vs. on the return vref for say the same wt is about 30% above vs. assume a stall speed of 180 kts so we are talking about absorbing the energy equvalent of 18 kts

Actually, no, because the worst case is where V1 was restricted by Vmbe, not by Vr/V2.

If you were forced to have a low V1 due to V1max being restricted by Vmbe then it's possible that V1 might be 60-70% of V2, in which case your Vref will be substantially above the Vmbe.

The only good thing about such a scenario is that if you were able to accept such a low V1 then it's likely you have a very long runway, which will help when it comes time to land.

[rhovsquared]

from Geapins "vr which inturn cannt be greater than v2, which by the way is about 20% above vs"

V2[depending on the airplane it it's cert.] is either

1. 1.05 *Vmu if tail grounding limit or elevator power limited
2. 1.10*Vmu if not tail ground or elevator limited
3. The speed that allows the greater of either 1.1*Vmca or 1.2Vso to be achieved at the applicable screen height

though they are all fundementally related to Vso



Keep The Brown Down
rhov

[rhovsquared]

Whoops the above applies to Rotation Speeds
V2 =1.1 Vmca or 1.2 Vso...sorry




Keep It Straight
rhov

[gearpins]

I agree M F S "If you were forced to have a low V1 due to V1max being restricted by Vmbe then it's possible that V1 might be 60-70% of V2, in which case your Vref will be substantially above the Vmbe."
that certainly puts a crimp on the margins Another thought though the Vmbe limts are demonstrated with worn brakes (only 10% of normal efficiency;JAR/FAR) .........got to run

[Intruder]

At max gross takeoff weight (377T), a 747-200 reaches Vmbe at about 130 Kt for a rejected takeoff. For a "normal" landing, it is about 190 Kt at max landing wt (285T) and 185 Kt at 300T. The chart I used doesn't go above 300T for landing, but the 130 Kt figure may be in the ballpark for a quick return after takeoff.

I agree that for a quick return at high gross weight, slow down NOW, and run off the runway at as low a speed as possible if that is inevitible.

[Mad (Flt) Scientist]

I'd say that the braking plan-of-action if you know you're going to exceed brake energy limits is rather type dependent. One type I was involved with a long time ago had no nosewheel steering, so a big concern if landing above brake limits was keeping enough brake efficiency to keep the aircraft straight once the rudder lost effectiveness. So I'd say if you have no NWS (either by design, MEL or due to failure) and have a Xwind you might have to try to keep some brakes to at least go off the end, not the side, of the runway.

if you do have NWS it would seem to make sense to use as much of the brakes as you can, but it still makes sense to delay if you can to maximise your aerodynamic braking contribution.

And I agree, with new brakes you're better off; but in today's economy, who thinks brakes aren't replaced at the last minute ....

[rhovsquared]

But, TO V1 is not limited at MTOW by Vmbe generally, Unless at high density altitude, so I think at lower DA's you may still have a margin even in an emergency scenario, I can't say where on a particular WAT curve...Also as was said, test were done with worn brakes and no RT so hopefully the holes don't line up like that




Happy Chandelles
rhov

[hawk37]

thanks for the replies....

Originally Posted by Intruder

It depends on the runway available... Specific procedures will vary with the airplane,

As the original poster, I was hoping for an Airbus or Boeing specific procedure for a case when the pilot chooses to land over Vmbe.
Alex (Viktor aircraft RAF) implies land and wait for Vmbe was his procedure.
MFS doesn't give a procedure, I suspect none is specified for him.
Intruder suggest slow down NOW, and run off the runway at as low a speed as possible. Did you suggest there is an AFM landing distance for 747's when landing over Vmbe?

[Intruder]

No. I meant to suggest that you may not have a choice of landing runway/distance if you HAVE TO land that hot & heavy. If you don't HAVE TO, there will likely be enough time to get a phone patch with Dispatch (and Boeing Tech Group, if necessary) to help you make up your mind.

Please note the scenario i presented in my first post. That is the one where you can ONLY do a 90/270 and land, or a VERY short circuit to the longest/most wind advantaged runway. The only info you may have at hand is the limit gross weight for landing on that runway, which will give you an indication of how likely you are to run off the end, and/or a computer-calculated landing distance that will tell you the same thing.

[Spanner Turner]

Would it be possible to get the 'self loading freight' to put their feet out the bottom of the fuse and assist in braking? - just like the Flintsones. I'm a ground engineer and only once have I had the (mis)pleasure of having to change 16 wheels and brakes on a 747 - anything you blokes can do to avoid this is worth doing! In this scenario it quite often follows that an evacuation is set in motion and then I have to change 10-12 slide rafts as well. On second thoughts, I guess this is better than being thrown a shovel to dig a 200 ton aircraft out of the dirt! Go for the lot I say - bury your feet in the brake pedals, spoilers to full as you flair, and hold max reverse until stopped(engines surging or not). You guys do a fantastic job.