V1, Vr and V2 question

[enigma]

BTW V1,V2,VR only applies to Part 25 Aircraft. All the little birds are certified under Part 23, these fall into two categories depending on weight, one requires a climb gradient with one engine out, the other just has to maintain a certain altitude with one engine. I think flight schools confuse the issue as they want there people to think they are preparing for the big time. People need to grow up and call a spade a spade.

Turbo. We may have the start of another thread here. I wasn't aware that flight schools were teaching incorrect/confusing stuff, but I'm not surprised.

Are some schools trying to use V1, etc, when flying non-V1 airplanes? If so, what other false material/ideas are the teaching in other areas? It's been almost twenty years since I started in a college flight training program, and 15 years since I taught in said program, but at Central Texas College we certainly didn't intentionally teach anything, or use any procedures that weren't technically correct. I can see where a school might want to utilize the concept of V1 in it's multi program, but I would assume that they would also correctly explain the term and explain that while they (the school) was using the concept that they were not in an airplane that used V1. If not, no wonder we have to deal with all of these know it alls (comment pointed at no one in particular).

regards,
enigma

 

[bigsky]

I do agree the king air is not a good airplane to discuss v1 as it doesnt apply 100%. And just because you are at v1 it doesnt mean you couldnt stop on ry available. I am sure on a 10000 foot ry you could climb to 50 feet and land and then takeoff again also. HOWEVER in that case you are not talking about a balanced field in which v1 becomes much more critical.( very seldom are you on a true balanced field-unless you are ry and weight limited or if reducing thrust and are temp limited I believe you have essentially balanced the field)
There is a big difference when discussing large turbo jet aircraft(king air excluded).
If you look at the stats there have NOT been very many high speed aborts at or above v1! Jets make better airplanes than land buggies! I routinely see takeoffs at weights in excess of 600000 lbs and when we get to v1 I am thinking to myself "those airport analysis people must be smoking weed-there is no way we could think about stopping"
V1 also has 2 meaning- max speed at which an abort has to be initiated to stop on ry and (b) minimum speed at which you could lose an engine and continue takeoff and still meet all the performance and climb restrictions. Thats why it is not a good idea to mess with v1...I have heard about some pilots who have arbitrarily lowered v1 to give themselves a little cushion on stopping- however now if they decide to continue?

 

[CLCAP]

Even on a 12000 foor rwy it may not be advisable to abort after V1. Maximum brake energy limits are also taken into account when V1 is determined (high gross weight) and you may end up with no brakes doing 40 kts of the end of the rwy.

V1 is usually determined by balancing the field length. (Acc go = Acc stop distance) so that the manufacturer can show the most favorable numbers as far as the airplane is concerned.

I think the test pilots have a 2.5 sec break (they're not allowed to touch anything) after V1 and then either abort or continue in the certification process.

Also - flying faster than V2 (if you lost an engine that is!!) can have dire consequences if your departure is predicated on a climb gradient.

As far as Vmu (minimum unstick speed) goes - I thought that that was the lowest speed when the airplane would "fly" and had nothing to do with striking the tail.

 

[Snoopy58]

V1 also has 2 meaning- max speed at which an abort has to be initiated to stop on ry and (b) minimum speed at which you could lose an engine and continue takeoff and still meet all the performance and climb restrictions. Thats why it is not a good idea to mess with v1...I have heard about some pilots who have arbitrarily lowered v1 to give themselves a little cushion on stopping- however now if they decide to continue?

Let me agree with your main point & disagree with what you said...

"Dont' mess with V1." I agree. Being a test pilot without any engineering data to back you up amounts to "using the force," and that doesn't strike me as a very good idea for most of us. V1 gets determined using a LOT of factors (many of which have been touched on at some point in the discussions above), and arbitrarily mokeying with it is setting yourself up for some very uncomfortable questions from an FAA type. Or worse!

OTOH, V1 might be the max speed at which you can safely initiate an abort, and it might be the min speed at which you can lose an engine & safely continue the takeoff, and it might be both (i.e. exactly balanced field in front of you), but it might be neither.

Take a lightly loaded small jet of your choice on a 15,000' dry runway with a good headwind on a cold day. Great performance, and, as you (BigSky) pointed out, V1 isn't much of a player -- you will get to rotation speed LONG before the point you need to put on the brakes to safely stop at the end of the runway, and you'd be able to continue the takeoff on one engine long before you reach Vr. In that case, V1 is neither of the above.

In a "stop-oriented" airplane, say the C-130, you always made V1 (though we called it something else in the Air Force) as high as we could. It was either rotation speed, or refusal speed (i.e. accelerate on 4, lose one, and stop on the last brick). As long as you could fly on 3 at that speed (and you pretty much always could), you went with it. (and if you couldn't, all you could do would be reduce weight or increase thrust by going bleeds off) Unlike in fast jets, we never reduced V1 in deference to brake cooling or such things. Heck, we had lots of reverse & beefy brakes! (insert manly Tim Allen grunts here)

In a "go-oriented" airplane, say the T--38, you virtually ALWAYS had a significant gap between V1 and VR, just because the jet flew pretty well on one engine (afterburners are wonderful things), and regardless of how much runway you hed in front of you, absorbing the energy of a T-38 at it rotation speed was awfully rough on the brakes (and consequently, dangerous) if you didn't have a really stiff headwind.

Other airplanes are somewhere in the middle -- like the 737. Frequently, V1 = Vr; sometimes it is slightly lower, other times (contaminated runway) it's a LOT lower. And it can be driven lower by things other than just accel-stop distance. Anymore, there are enough factors going into the number-crunching computers that you can't always say for sure what the controlling factor is.

Usually, there is a range of speeds below refusal (accel-stop) and above decision speed (accel-go) where you -- or really, the engineers who write your takeoff data tables -- can set the V1, and you'll be able to stop at or below V1, and also be able to fly at or after V1. (And, live with the brake temperature if you stop.) For instance, if Vr is 130, you may be able to fly anytime after 110, and you can stop up until 125. Where's V1? In a Herk, we'd say 125; in a T-38 probably closer to 110 or 115. Where you put V1 in that range for a particular aircraft is a choice. Sometimes that choice is based on factors such as the knowledge that, on the one day in 1000 when you actually face to go/stop situation, you may not exactly duplicate the test pilots' performance. As one performance engineer put it, if you lose one and then cross the departure end at 34' instead of 35', nobody will care. But if you're a little slow getting on the brakes and you stop with the nose tire at 7001 down a 7000' runway, your day just got a lot worse!

So, in simple pilot terms, V1 is the speed by which you make your decision. Until V1 I can decide to stop, after V1 I need to go. At Vr I rotate. After that, if I'm flying with a problem, I want V2 at least. V2 may be predicated on Vyse, or Vmca, or other things. (for instance: Herk V2 went UP with increased thrust -- worse asymmetric thrust if you lost an outboard; 737 V2 goes DOWN with more thrust -- more thrust to balnce the greater induced drag at low speed) Prior to V1 you MAY be able to fly away safely, but it's a better decision to stop. After V1 you MAY be able to stop safely, but it's a better decision to go (if you can. If not... "hit the fence fast, or hit the fence slow" applies). In any event, as you're rolling down the runway, you need a simple decision point, not lots of factors to think about. And V1 is that easy simple decision for you. All the hard thinking has already been done by the engineers.

So I'll repeat my agreement with your main point: don't go "adjusting" V1 arbitrarily!

 

[antney]

Flychicago I'll check that out.

Also those runway analysis charts are numbers and a lot of is in theory. Wait till to you take off from Midway on a 95 degree day, full boat in the Saab, and when your calling V1, you tell me if you would have enough runway left to stop with normal braking, meaning no reverse. It's one thing to read the numbers, its another to be calling V1 and already knowing that you were taking off before you called it. You'll love seeing that saab rotating at the touchdown markings on the opposite side!
Anthony

 

[Loafman]

Just a sidenote to the V1, go/no go discussion, there may be additional circumstances you want to take into consideration when briefing aborted takeoff considerations. Short or contaminated runways or low visibility may dictate which abnormals/emergencies you abort for and those that it would be safer to continue the takeoff even prior to reaching V1. In those cases, a 70 or 80 knot call would define the point at which you'd only abort for engine fire/failure or aircraft controllability problems.

 

[Snoopy58]

enough runway left to stop with normal braking, meaning no reverse.

I think most V1 charts assume max antiskid braking, which is a whole lot beyond what most of us ever feel on landing, even at a place like MDW.

But you're right: loaded to the gills with "170#" pax on a hot day at a short runway, you're on the edge of just enough performance to make it work if you lose one at just the wrong moment. Enough, but not a lot more.

 

[TurboS7]

Good discussion. V1 is a hard number it shouldn't be compromised on a FAR25 aircraft. There are many approved ways to "slide V1" they will be in the appropriate performance manuals for your operation and aircraft. The bottom line is that if the book says it will fly it will. In the case of the DC-10 they had lost all their hydraulics and an engine. Usually performance guys don't figure that into the equation. The DC-10 didn't have an engine failure it had a catatrophic failure, in which case only the wits and wisdom of the flight crew will bail it out. Something major going wrong demands something major in the other direction to offset it. In the case of the DC-10 going for the speed would have saved it. Now we come to negative training in the sim, we teach for what we think will happen not for what will. Now days with the type of engines that we have if there is a failure usually something really bad has happened, that needs to be evaulated before slugging into emergency procedure. Our engine failure checklist is not by memory anymore, we evaulate it and work our way through it.

 

[enigma]

Our engine failure checklist is not by memory anymore, we evaulate it and work our way through it.

Same here. The Engine Fire/Sever Damage/ Separation Checklist is still a memory item, but not the Engine Failure.

regards,
enigma

 

[SquirrelDog]

Actually VR can be lower than V1 according to the charts of our Aircraft. In some instances, due to temp, airport elevation, runway length and use of flaps VR is lower. This does not mean we rotate at VR and then make the decision at V1, we just move VR up to V1 to make it the same speed. It is only a few knots lower anyway in this rare situation.

Loafman makes a good point, in which making a decision early in the acceleration process, about only aborting for Engine failures after a "said" speed for critical issues. That is why a proper pre-take off breifing is important for each takeoff and the different conditions.

BTW isn't a balanced field also one that at higher elevations such as 7000 msl has a runway 7000 feet or longer. Something I heard awhile back.

Oh, if I lose 1 at VR 3500 down a 13,000 foot runway at 8,500 msl with a 85F outside temp, The B#tch is staying on the ground Screw the Brakes, I got several sets anyways! Been there done that.

Fly safe Squirreldog

 

[SkyWestCRJPilot]

For those of you advocating keeping the airplane on the ground after V1 if you have plenty of runway, I don't know what type of airplane you are flying and perhaps in certain circumstances you are correct. But in the CRJ which I fly there is zero chance I would abort after V1 no matter what. After V1 the airplane is a flying machine and flies better than as a high speed land machine. Even if the runway were 50,000 feet long I wouldn't abort after V1 with an engine failure. I know beforehand that we have plenty of performance on the other engine and we've trained for it (if you don't have the performance you shouldn't be taking off at that weight). It is much safer to takeoff and deal with the emergency in the air than try to slow a fast moving turbojet on the ground. Tires can blow and catch fire, directional control can be lost, and you can kill a lot more people that way than by just taking off and coming back around after completing the memory and emergency checklist items. When I flew the Chieftain I would consider landing on the remaining runway after takeoff if an engine quit. I don't know what type of airplane those of you are flying that advocate aborting after V1 but I say no way for any turbofan/jet.

 

[bigsky]

If my memory serves me correctly(maybe does-maybe doesnt) but the DC-10 in Chicago had the whole engine rip off the wings at the pylon-- destroying and retracting some of the slats when it went... that I believe is the main reason they lost it when they pitched up to V2.
The DC-10 then sent a software fix so on engine failure the pitch bar would maintain either v2 or what ever speed it was currently at(the higher of the 2 within reason)

 

[enigma]

Oh, if I lose 1 at VR 3500 down a 13,000 foot runway at 8,500 msl with a 85F outside temp, The B#tch is staying on the ground Screw the Brakes, I got several sets anyways! Been there done that.

You're missing the point. Brake energy limits have nothing to do with brake maintenance, they have to do with performance. If you have 1000 units of energy to disapate, but your brakes can only absorb 800 units, you will not stop. If you abort in said situation, you will find yourself moving down the runway unable to stop and fight the fires eminating from the truks. Not a real nice situation. When you do finally coast to a stop, the pax will initiate the evac on their own, and at least two major injuries will occur in the evacuation. You will then be doing the carpet dance with both the FAA and in the CP's office explaining why you choose to disregard standard operating procedure/recognized safe procedures.

something to think about.

regards,
enigma

 

[blott]

But in the CRJ which I fly there is zero chance I would abort after V1 no matter what.

Even if the runway were 50,000 feet long I wouldn't abort after V1 with an engine failure.

"NO matter what"? how about if both wings fell off? I can see it now...wings laying 1000' behind you on the runway, you past V1, pulling back...at least you could say "I died trying" , then again, I guess you couldn't say that.

50,000' and you still aren't going to abort? You are a smoking hole in the ground waiting to happen-think about it...

B

 

[100LL... Again!]

The following is from one of my earlier posts.
Search under my username to see the replies.




Warning - the following post contains no references to Delta, Comair, PFT, the RJDC, Freedom, or airline management. I.e. BORING!
------------------------------
To answer your question-

There is no real easy way - you might try that book that was mentioned.

Also, I have heard of a book by a European pilot called "From Takeoff to Landing.' If you find it, let me know - I haven't tried yet.

Some knowledge of the Terps is helpful. Specifically, what gradients are, how they are used to compute takeoff performance, but here is what I got off the top of my head:

Correct me if I got this stuff wrong.

Segt1- Liftoff to gear retraction, assumed to be 35' over the end of the runway.

Segt2- Initial Climb at V2. This segt is complete at 400 abv the runway, or the acceleration altitude. Most are 400'. Obsacles can dictate higher. Roanoke, Aspen, etc.

Segt3- Level off and accelerate to flap retraction speed. Retract flaps.

Segt4- Climb to 1500' abv rwy elev. at whatever speed the manual dictates. V-whatever, varies among aircraft.


Vef- The speed at which the engine is assumed to fail. (Basically V1 minus pilot reaction time.)

V1- The speed above which a takeoff will be performed regardless of engine failure. Depending on weight, can also be the max speed from which an accelerate-stop can be accomplished (on the runway).
At low weights, there are a range of possible V1 speeds, limited on the upper end by stopping ability. Limited on the lower end by one engine acceleeration ability.

Example: Take a 727 at a light weight. If v1 is too high, an abort would take you off the pavement even if done before V1. If V1 was (for sake of argument) 10 knots, the airplane might not be able to accelerate to Vr before running out of runway.

As you get heavier, the spread between the upper and lower limit converge. (KINDA like Vy and Vx do).


Vr- The speed at which a normal rate of rotation will produce V2 at 35' abv the runway. (not necessarily the runwway end.)

Vmu- Min unstick speed figures, in but can't remember how off the top of my head.

V2- takeoff safety speed. A VERY ROUGH analogy would be Vx. This speed is designed to get you over the obstacles. It is limited by (among other things) Vmca and Vs.

I think the factors are 1.1 Vmca and 1.2 Vs.
Therefore an airplane with tail mounted engines would probably have V2 limited by Vs, since there is little assymetric thrust. Wing mounted engines could go either way. If lots of power is available and the weight is low, 1.2 Vs might be lower than 1.1 Vmca.
If the weight is high, 1.2 Vs will probably be more limiting than 1.1 Vmca.

Vs is stall speed for the takeoff configuration being used.
Vmca is what you think it is. Air minimum control speed.

There are several (depending on aircraft) V-designators for the speed at which the flap retraction BEGINS, as well as the fourth-segment climb speed. Check the manual.


Vmcg- Ground minimum control speed. This is the speed ABOVE which, when an engine is lost, control of the aircraft can be maintained on the ground USING ONLY AERODYNAMIC CONTROLS. No nosewheel steering. "Control" means being able to keep it within 30' of the centerline after an engine failure. Yee-Ha!

Again, off the top of my head, I can't remember if this speed is associated with V1 (my bet) or Vr/V2.



Climb gradients (not rates) are associated with 2, 3, or 4 engine aircraft, and affect how much performance is requied for each engine out condition.

Also, be careful, some gradients are published as percent slope, some as feet per nautical mile. NOT interchangeable - math required.

I worked out the formulas if you want them, along with some common gradients and slopes.


'scuse me now, I'm going to go try to get a life...




Oh yeah, read part 25 (also the perf. parts of 121) a couple times. It ain't that bad. Really.