Hi,

 

regarding the dual engine failure after V1...this is what you do:

 

 

:-)

Frederik

Well keep in mind that it all depends on what V1 is based on. Over the range of aircraft, V1 could be based on critical field length, min ground control, critical engine failure speed, max brake speed or even tire limiting speed in the heavies. Gross weight and conditions will cause these speeds to vary. Runway only becomes a factor when the runway limits the aircrafts weight. Lets say you did data for a 5000ft runway and that length limits you to 135,000 pounds. At this point, your critical field length equals runway available and you are field length limited. If you reject above V1 in this scenario, you will depart the paved surface. If the runway is longer than your critical field length, you may be able to stop. At some point you will exceed max brake speed or tire limit speed if you keep it on the runway and reject. Above max brake speed, you brakes have reached max absorbtion and will lead to a brake fire. Above tire limit speed, the tire literaly rips apart. Good question!

 

This pretty much covers it. V1 can be limited by runway length, climb performance, brake energy limits, vmcg. Vmcg constrains V1 at the low end (you can't continue with TOGA thrust on one engine at less than the speed at which you can keep straight with rudder and nosewheel steering), nor can you go unless the remaining TODA at the point of failure is enough to reach 35' by the end, at the high end of the V1 calculation you cannot stop unless the brakes and tires can cope, nor can you reject after rotating.

 

You actually end up with a range of possible V1 speeds, at the bottom of that range you can only just go, at the top of that range you can only just stop. Below the bottom of the range you cannot go, above the top of the range you cannot stop. From that possible range of V1 speeds, you'd then pick one right in the middle ideally, since that would mean either a reject, or a decision to continue, made near V1 would have a margin of error, however in reality you'd normally increase weight (or derate thrust) both of which make it harder to continue and harder to stop, so this narrows the range of V1s (by increasing V-min-can-go and decreasing V-max-can-stop) until you end up with a single V1 at which you can barely stop and barely go.

 

If you want to see a field-length limited stop, you'll need to do your calculations properly with TOPCAT to ensure you have a field length limited situation. When you do that, you will find that the modelling is actually pretty accurate, and you really have to be on the ball to stop on the runway - V1 is not actually the "decision speed" although it's often referred to as that, it's actually the speed at which you have to take the first action to stop, so the decision must actually be made slightly before V1. You then have to get the throttles closed, speedbrakes up, reversers at full, max braking all rather quickly to stop.

 

V1 is also based on net performance (ie. an average) so it's possible that your aircraft won't actually stop, if I remember rightly there is no factoring (no margin of error) in V1 calculation, because it's already considered sufficiently unlikely that you'll try to initiate an RTO exactly at V1 (and max v1 from the range at that), on a limiting field so no additional factoring is required.

 

In the go case, you do have factoring in the climb performance calculations, so an RTO near V1 is far more dangerous than continuing to go. In most cases (ie. a simple engine failure) it's much better to continue given that you have a perfectly flyable aeroplane, than attempt to stop.

 

If you don't have TOPCAT, here are a couple of example situations to try, which are field length limited so should test your ability to stop on the runway:

 

Situation 1:

 

EGMD (Lydd), RW21, departing to Innsbruck (LOWI). WX at Lydd 210/29, +11, 1007, dry

118 pax on board, 1534kg of luggage

5815 kg of fuel on board

TOW 59343kg

Flaps 25

Derate to TO-2, with a flex temperature of +33c which gives you 87.7% N1 (according to TOPCAT, but the thrust normally ends up very slightly different in the NGX, not sure why)

V1 = 133, Vr=133, V2=133

 

Situation 2:

 

Same as above, but with a wet runway.

Flaps 25

TO-2, no flex

V1 = 120, Vr=133, V2=133

 

Notice that Vr and V2 are the same in both these cases - because the only difference with a wet runway is that it's harder to stop, therefore V1 must reduce (for the same weight) but the speed at which you rotate, and the TOSS remain the same.

 

Example 3:

 

LOWI, RW26, WX is 270/08, +2c, 1023, wet runway

160 pax, 7286kg of baggage, 6415kg of fuel on board

TOW 69027

Flaps 25, no derate (TO thrust setting, no flex)

V1=131, Vr=143, V2=143

 

Example 4:

 

Same as above, except with a dry runway. This is now no longer a runway limited situation, but rather a climb limited one. That means you can put a bit more weight on board, but we're at MZFW, so we'll have to do it as extra fuel - bump the fuel up to 8375kg on board, then have an engine failure at V1 and continue - you'll just manage to maintain minimum terrain clearance if you fly it correctly. I can't remember how it's calculated, it's quite complex how the obstacle clearance area is shaped and what the requirements are within that area - it's all inherent in the departure procedure and OEI performance requirements for certification, so as a pilot you don't actually need to know it in detail.

 

V1=141, Vr=142, V2=146

What if both engines fail at Vr on a twin jet?

 

Not a bad quetiosn.

 

Then I think that is when pilot instincts come in to play. With all your brakes and spoliers you would have slow down using the clearway of the runway.

 

The probabibility and reason for it happening would be quite unlikely. A flock of birds for example. The liklihood of two engines failing machanically on their own is very unlikely. Unless the fuel was pollutted in some way.

 

I would be more worried about moments after VR...then you really have got to pick the next river or road to ditch the plane in/on.

Not a bad quetiosn.

 

Then I think that is when pilot instincts come in to play. With all your brakes and spoliers you would have slow down using the clearway of the runway.

 

The probabibility and reason for it happening would be quite unlikely. A flock of birds for example. The liklihood of two engines failing machanically on their own is very unlikely. Unless the fuel was pollutted in some way.

 

I would be more worried about moments after VR...then you really have got to pick the next river or road to ditch the plane in/on.

 

It would be extremely unlikely for both engines on a twin to fail simultaneously at Vr. The only possible reason would be massive indigestion caused by birds or a fuel flow problem. If it were to happen then at least the fire crews would get to the a/c remains quickly. But, practice your engine out drills meticulously. I had a double engine fire in my VC10 yesterday :-(. A two engine out scenario is quite a heavy workload notwithstanding that I had to freefall the gear, that took three minutes!

On the 737, V1 and Vr are usually a few know apart, unless you're using wet data, then they might be 10 or more.

 

If both engines fail at Vr, you're not going to accelerate, so you're not going to do more than a small hop, and only if you yank it into the air. And why would you do that?

 

This is trending towards the treadmill discussion. . .