I wonder about one thing. Supposedly V1 is the speed at which you are dedicated to taking off. In order to know where this point accurs, one needs to know the length of the runway. I guess this lies within the FMC database as it's not fed to the system in any other way. For fun I just attempted to reject a number of takeoffs above V1 speed with wet runways and in all occasions I stop with quite some runway to spare. Don't know how well this part is modeled by PMDG. Or maybe this is the way it should be?

Rwy lenght, weight & load ?

It probably has to do with balanced field takeoff calculation PMDG uses that has been mentioned before in the forum. This basicaly means that V1 is calculated so that TODR and ASDR are the same for a given rwy lenght and aircraft load and configuration. The lowest value is usually applied so that in case engine fails at V1 you have enough distance to stop the aircraft OR to continue with takeoff and bring the aircraft to 50 ft above the runway at its end with one engine inop. In some cases TODR will be higher in others ASDR will be higher. If you've got plenty of runway left it just means that V1 for that specific runway, flap and thrust config as well as aircraft load was computetd based on TODR.

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!

You're also probably using brand-spanking new tyres and brakes. V1 doesn't assume that, since that's best case. Worn brakes get hotter faster, so are less effective.

Good post Rick.

 

You only forgot one thing about V1, which is quite often the case with the 737 - must not be higher than Vr

Regarding this topic, I have a question of my own (I dont mean to hijack the thread - I think its up the same ally). What if both engines fail at Vr on a twin jet? Isnt it better to keep it on the runway and face whatever may come, rather than to try to lift off and maybe crash in houses?

 

PS! I know its like astronomically unlikely that both engines will fail at the same time - but what IF?

Then you go in the Hudson

Basically, I would say stay on the ground hit everything brakes, emergency brakes, spoilers, flaps, reversers if they will even without engines just for a little more drag, anything that will help stop,

I don't think i would try and bunny hop anything.

Then you go in the Hudson

 

Doubt it...the Q was ...

 

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

 

Then you probably try & put it down immediately facing the consequences with every slowing device in you armoury deployed pulled or depressed as you steam toward the RWY approach lighting poles immediately ahead...you probably don't have enough energy to do much else.

 

Hudson excursions require at least a margin of altitude & speed combined with a clean or almost clean aeroplane & a fairly wide expanse of fairly calm water adjacent to you flight path...you have neither the first two...the aeroplane is dirty...so the Hudson is irrelevant.

wht kind of question is that anyway? As soon as you got no thrust, you go down from your present position, not up, and it is not exactly up for discussion....

I don't think it was meant to be taken literally (the Hudson). At Vr you're flying, no matter what (as you are at V1). If a nice wide river is available that's where you go. If not, just make the most of it. You don't go down until you run out of energy to stay up.

An important thing to remember when you back away from the details, is that V1 is a "guarantee" speed--if you reject at/below V1, you are guaranteed to be able to stop in the remaining runway (and stopway, if one is available), and if you lose an engine and continue at/above V1 you are guaranteed enough performance to take the jet into the air and cross the departure end (plus clearway if available) at 35' AGL. If the runway is 12,000 feet long at sea level, you very likely can stop safely from a reject initiated at speeds well above V1. But you're not guaranteed that, and in a time-critical situation like deciding on a takeoff abort you can't take time to do that what-if drill. So we do the analysis beforehand, and decide on a decision point that guarantees us a stop option below that point, and a go option above it.

 

But if the problem is something more serious than a simple engine failure--like an uncommanded thrust reverser deployment, for example, one might decide to reject at any speed and take their chances on the ground rather than an almost certainly fatal outcome if takeoff is continued.

 

Regards

At Vr you are not flying yet, you are flying at couple knots past. Still, unless you want to do a bunnyhop, no reason to pull up?

Thomas, can you provide details on the specific examples you are asking about -- Airport/runway, specific airplane model, weight, flap setting, thrust setting, temperature, and winds? Did you use reverse thrust during the stop? Although I have my doubts about how closely an FSX NGX sim can match the airplane's certified takeoff performance, I expect the answer will be simply that you were probably no where near a runway length limiting situation. if the runway is much longer than the minimum that you need for your takeoff performance, you will be able to stop within the runway length from well above your V1 speed.

 

You also mention you did this on a wet runway. Does the FSX NGX braking model account for runway surface conditon in any way?

 

Don Stimson