V1 vs Vref

[uwochris]

Hey guys,

Can someone please explain the difference between these 2 speeds? I am reading a book in which the definition of each speed is very similar.

My understanding of V1 is that if you go beyond this IAS, you cant stop in the available runway. This speed should depend on a/c weight and runway length (amongst other factors, like friction index, etc).

TIA,

Chris.

 

[fatburger]

V1 is go/no go speed in case of major malfunction. Vr is rotation speed on takeoff. Both vary depending on weight, weather conditions, field elevation..etc.

Vref is approach speed for given weight and flap setting.

-fatburger-

 

[CitationLover]

V1 is also the speed at which following an engine failure at the critical moment the airplane can takeoff and achieve the required height above the takeoff surface within the takeoff distance.

Vref is simply 1.3 Vso. FAR 1 defines it as landing reference speed. It is adjusted by weight, flap settings, and winds (gust factor, etc).

 

[SCT]

Chris,

You are getting Vref and Vr (rotate) confused. Vref is a landing speed and Vr is the rotation speed on takeoff.

Also, V1 is the Take-off decision Speed. Yes, if you lose an engine right before V1 you will have the required distance to stop the airplane. BUT more importantly, if you lose an engine at V1, you will have the required takeoff distance to accelerate to vr on a sigle engine and then rotate and climb to 35 ft (minimum) over the end of the runway. The numbers are all a factor to weight, altitude and temp. This is what the a/c is certified to do.

Now V2 and second segment climbs, you could write a novel about.....

 

[LimoDriver]

There are different ways to interpret V1 depending on the airframe. I think the most common is: After V1 you cannot stop within the balanced field length - not necessarily the runway length. Of course, the available runway has to be equal to or greater than balanced field length or it is an unsafe takeoff condition.

But, equally important (and people tend to forget this part), if you lose an engine (in a multi engine airplane, of course) prior to V1, you cannot accelerate to Vr within the balanced field length. Or, more precisely, you cannot accelerate to Vr and be at 35 ft over the end of the balaced field length.

Comments?????

 

[uwochris]

Thanks for the replies.

In "Flight Theory for Pilots" by Charles Dole, he defines Vref as: "Refusal Speed: the max speed that the a/c can obtain under normal acceleration and then stop in the available runway. Vref applies to single and multi engine a/c."

He also mentions that V1 applies only to multi-engine a/c, and must never exceed V1.

It seems odd that Vref is also defined as an approach speed (reference speed for approach).

Chris.

 

[Snoopy58]

Chris,

Most of the world refers to "Vref" as a landing speed, i.e. "reference" as you noted. The guy whose book you're looking at is using notation different from most everybody else these days, and that's what's caused the confusion. Not an invalid way of looking at life and performance and such, but a rather nonstandard notation. The way the Air Force taught takeoff performance, you had a "decision speed" which was really a MINIMUM speed at which you could continue a takeoff having lost an engine, a "refusal speed" which was the MAXIMUM speed at which you could stop on the runway available, having just lost an engine. If "decision speed" is below "refusal speed," life is good, and if you lose one between those two speeds you should be able to successfully execute an abort but also you could successfully take off. If "decision speed" is ABOVE "refusal speed," then life is very bad, because if you lose one between the two, you can neither stop nor go... that's not good!

The Air Force wouldn't let you fly on days when "decision speed" was above "refusal speed," and (not wanting their fledgling pilots taking bad aircraft into the air unnecessarily) told you that until you reached "refusal speed" you weren't committed to the takeoff. They didn't use the terminology "V1," but if they had, they'd have set it = "refusal speed." That's not a universal preference, by the way... if you have a range of speeds where the aircraft can successfully stop AND successfully go, some operators will set their V1 to a value below the maximum "refusal speed" since they figure that stopping on the last brick with smoking brakes is more dangerous than taking the jet airborne... particularly since nobody notices or cares if you had 34' of obstacle clearance instead of the engineers' 35', but EVERYBODY cares if you go one foot into where the approach lights are set up!


Limo,

You're not wrong in anything you said, but bringing balanced field into it complicates life tremendously. V1 is your (make a) DECISION SPEED: before V1 you can stop the airplane, after V1 you won't try to. Period. (note: this is NOT the same as what the USAF called "decision speed"!)

Doesn't mean that on this runway with today's conditions you couldn't continue with a problem that occurs before V1, and it doesn't mean that you couldn't stop the airplane with a problem after V1. With a long enough runway, you can lose an engine well before V1 and continue the takeoff successfully; with a long enough runway you can initiate a stop well above V1 and do so successfully. If the runway is long enough, there's no reason you MUST base V1 on anything related to balanced field whatsoever... if you have enough performance, you (actually, whoever has done all the performance calculations) can set V1 = Vr, thus not having to commit yourself to taking a bad airplane airborn when there's ample room to safely stop. Or, on that same runway, a legal V1 could also be incredibly low... for instance, most 4-engine jets are approved to do 3-engine takeoffs for ferry purposes... so with 4 engines operating, their minimum speed to continue the takeoff should they lose one engine could be as low as 0 knots. Why would you WANT to have a V1 as low as possible, I don't know... but you could. (maybe, getting out of Dodge with bad guys shooting at you)

There is a LOT of stuff that can go into how the V1's are set and calculated and affected by winds and brake cooling and balanced field length and what wiggle room the performance engineers have to set it anywhere they choose within some range, but the simple pilot definition is, it's your decision speed. Before I get to V1, I can stop. Don't have to, but can. After V1, I won't try to. (Might be able to, but won't try, unless the aircraft is incapable of safe flight, in which case the choice is between running off the end of the runway really fast or somewhat slower, but that's beyond the scope of "normal" emergencies that get considered.)

K.I.S.S., V1 is the speed after which I'm not going to try to stop the airplane on takeoff. Questions?

 

[Dangerkitty]

Thanks for the replies.

In "Flight Theory for Pilots" by Charles Dole, he defines Vref as: "Refusal Speed: the max speed that the a/c can obtain under normal acceleration and then stop in the available runway. Vref applies to single and multi engine a/c."

He also mentions that V1 applies only to multi-engine a/c, and must never exceed V1.

It seems odd that Vref is also defined as an approach speed (reference speed for approach).

Chris.

I think Charles Dole is a little confised. Vref only applies to Approach. It has nothing to do with Takeoff. The above quote is the definition of V1 not Vref.

As far as V1 only applying to Multi-Engine aircraft, well that can be debated.

 

[Snoopy58]

DK,

I think what Chris meant to type was
>> He also mentions that V1 applies only to multi-engine a/c, and must never exceed Vref.
("Vref" here being his notation for "refusal speed," not the landing Vref that the rest of us know & use!)

Everybody has a "refusal speed," i.e. a max speed at which they can stop. In a single, up until that speed you can stop if you want to. In a single, however, there is NO speed after which you can lose one and expect to successfully continue the takeoff, which renders the idea of "... and after V1 I can ___" harder to define for a single!

 

[LimoDriver]

Snoopy -

I agree with everything you said except:

"Before I get to V1, I can stop. Don't have to, but can."

If you lose an engine prior to V1, and you elect to continue the takeoff, there are no guarantees you can be airborne in the remaining runway. Obviously, if it is a long runway, and runway available far exceeds balanced field length, you'll probably make it just fine. But, unless you've done the math ahead of time, you really don't know for sure. (Of course now ground minimum control speeds come into the equation, but that's a different subject.)

The air force used a speed called "critical engine failure speed (CEFS)" to determine the speed you had to accelerate to on all operating engines to safely continue the takeoff in event of an engine loss. Do any civilian operators compute this speed? I know we don't!

And if memory serves me correctly, we used a term called critical field length, which was the runway length where CEFS and Vrefusal were the same - basically balanced field length. As long as runway available met or exceeded critical field length, the takeoff was safe. It was a very confusing and convoluted approach to takeoff performance data. Very few pilots really understood it!

 

[typhoonpilot]

Chris:

You always ask very good and thought provoking questions on this board. The V1 and Vr question is excellent. There have been some good answers on this thread that are generically correct. There is an old definition of V1 and there is a new defintion of V1. It is important to note the difference. The old definition is, " Decision speed in the event of an engine failure on take-off; at which the take-off may be either abandoned or continued ". That comes straight out of Handling The Big Jets originally written in 1967. The newer and more refined definition came about in 1992 when the industry wanted to clear up confusion caused becuase it was not clear that V1 is the MAXIMUM speed at which the flight crew must take the first action to reject a take-off. The new defintion from the B777 manual is, " V1 means the maximum speed in the takeoff at which the pilot must take the first action ( e.g., apply brakes, reduce thrust, deploy speed brakes ) to stop the airplane within the accelerate-stop distance; and, V1 also means the minimum speed in the take-off, following a failure of the critical engine at Vef at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.

The other thing not mentioned here but very important in the go/stop decision is being very clear what you will stop for and what you will go for. Most newer generation airplanes are engineered so that they will not beep, squawk, or otherwise alert you if they don't want you to stop. On the B777 different levels of alerts are inhibited at different stages in the takeoff run. This leaves only serious concerns such as engine failure, engine fire, and predictive windshear active above 80 knots ( PWS even ceases at 100 knots until above 50 feet ). This is Boeings generic philosophy based on the Takeoff Safety Training Aid. Essentially we brief that we will only reject for an engine fire, engine failure, or aircraft unsafe/unable to fly once above 80 knots. The reason for this is that it is statistically much safer to continue with minor problems than to try an abort above 80 knots.

The actual answer to your first question is that the V1 speed can be exceeded and most likely will be if you initiate the reject right at V1. Remember it is the speed at which we take the first action to stop so momentum will keep the aircraft accelerating until the stopping forces start to decelerate you. When I taught on the MD-90 it was very normal in the simulator to see the maximum speed reached about 5 to 10 knots above V1 when I programmed in a malfunction just below the V1 speed. That particular simulator had a very neat page which showed reaction times for beginning of braking, thrust to idle, and maximum braking.


Our follow-up question of the day is, " If you lengthen the clearway what will that do to the V1 ? "


Typhoonpilot

 

[Snoopy58]

Limo,

As far as my remark about "don't have to {stop}, but can," I was referring to the sorts of lesser problems that Typhoon talked about being suppressed in the 777... do you abort for a master caution light at 50 knots? at 100? at V1 minus 5? Per the math, you COULD successfully stop the airplane at any of those speeds... but you don't have to, and it might very well be a bad idea (smoking brakes for a pressurization fault???). As far as a full blown engine failure, you're correct that before V1, stopping is the far better option since going may or may not be posssible, depending on some math that us pilots probably don't have access to. Below V1, you can stop for a problem (though depending on the problem, you may not want / need to); above V1, you'd better not.

Cheers!

 

[uwochris]

Thanks again for all the replies.

Typhoon, in response to your question... (and this is only a guess as to what the answer is!). Since a clearway is not designed to be constructed of a load-bearing surface, it does not affect your TORA/TODA or your ASDA. If it were a stopway, then it would extend them, and thus, effect V1. I am guessing that if you have more runway availble, then V1 can be increased. In the case where there is an extension of a clearway, it will not effect V1 at all.

 

[typhoonpilot]

Chris:

The TODA is useable runway plus clearway. The presence of a clearway allows you to take-off at a higher weight, but since the runway length ( pavement ) remains the same you must decrease V1 to stay balanced.


TP

 

[uwochris]

Thanks Typhoon,

Another question though... if a clearway is not constructed of a load-bearing surface, why does it affect your TODA?

I initially thought a clearway and a stopway were the same things, but now I realize they are not. I was told that a stopway (marked with the yellow chevrons) should not be used under normal circumstances, although it is constructed of a load-bearing surface, and can lengthen the ASDA, TODA, etc.

I am not all to familiar with clearways, stopways, etc and how they affect your TODA, LDA, and ASDA. It just seems odd that if a clearway or stopway are not meant to be used under normal circumstances, then they should not affect the calculations (LDA, ASDA, etc). Why not just make the runway bigger in the first place, instead of adding clearways and stopways??

Chris.

 

[typhoonpilot]

if a clearway is not constructed of a load-bearing surface, why does it affect your TODA?

If clearway is available, the point where the airplane climbs to 35 feet can be over that clearway ( as opposed to the end of the runway ). This permits you to use a higher weight because you have more distance to climb to 35 feet.

I am not all to familiar with clearways, stopways, etc and how they affect your TODA, LDA, and ASDA. It just seems odd that if a clearway or stopway are not meant to be used under normal circumstances, then they should not affect the calculations (LDA, ASDA, etc). Why not just make the runway bigger in the first place, instead of adding clearways and stopways??

Another good question. One not with an easy answer though. I would guess due to either space or money constraints. If you can satisfy most operators needs with a 3000 meter runway that has a stopway or a clearway for added benefit then there really isn't the need to spend the money to build and maintain that extra 500 meters or so of runway/lighting/taxiways,etc. Also you may only have enough room to build a 3000 meter runway and no more.

Hope that helps,


Typhoonpilot

P.S. A good book on peformance can be had through the Transair pilot shop in the UK. It's called Understanding Performance by A.J. Walters

 

[mar]

Great thread

Sorry to join late.

One small point to make then I gotta run:

Clearways and stopways are two *physically* different things.

A stopway is a strip of pavement provided to stop the airplane and may be included in the acc/stop distance.

A clearway is a path of airspace free of obstructions that's used to continue to accelerate the aircraft after the takeoff is continued with a broken engine. This distance may not be included in the acc/stop distance because obviously you're not stopping, you're accelerating.

The *stopway* may not be used for normal operations because of something simple like they didn't want to spend more money on strengthening the pavement...so they just provide it for emergency stops.

I hope that was clear and concise. If not, I'm sure someone else will chime in.

Fly safe.

 

[uwochris]

Thanks again.

Here are some definitions I found (they are Canadian sources, but I am guessing they are the same as the USA).

Stopway: a defined rectangular area on the ground at the end of the runway in the direction of t/o, prepared as a suitable area in which an aeroplane can be stopped in the case of an abandonned t/o.

Clearway: A defined rectangular area on the ground or water under the control of the appropriate authority, selected or prepared as a suitable area over which an aeroplane may make a portion of its initial climb to a specified height.

TODA: the TORA plus the length of the clearway (the maximum clearway length that can be included is 1000 feet).

So I guess the clearway can lengthen the TODA, but NOT the TORA. The stopway can lengthen the ASDA, LDA, and TORA and TODA ( a part of me is telling me that the stopway does not affect the TORA, LDA, and TODA because it is not technically "suitable"... am I wrong here??).

Chris.

 

[Loafman]

I think Charles Dole is a little confised. Vref only applies to Approach. It has nothing to do with Takeoff. The above quote is the definition of V1 not Vref.

As far as V1 only applying to Multi-Engine aircraft, well that can be debated.

so if you lose an engine after V1 in a single, you're going to take off anyway?

 

[minitour]

so if you lose an engine after V1 in a single, you're going to take off anyway?

...in a single, wouldn't V1 = Vr?

I'm thinking if the runway is short enough that you have to make a decision, you had better be airborn VERY shortly after that time...

-mini

 

[Snoopy58]

Why not just make the runway bigger in the first place, instead of adding clearways and stopways??

$$$
Anybody priced out the cost of pavement recently? Runways are expensive compared to the cost of removing a few obstacles.


V1 involves a decision... before this speed my decision is to stop if I lose one (and I can stop for other things if I need to), but after this speed my decision is to go (unless the plane is unable to fly), even having lost one. In a single, when you lose "one," there IS no decision to be made. REFUSAL speed is still a player, since there are other things you might like to stop for (if you still can) other than a failed engine... until refusal speed you can stop in the runway remaining, after refusal speed you can't. Refusal speed deals with stopping only, and thus applies to a single. But a speed that deals with stopping OR GOING such as V1 doesn't apply to a single.

 

[Loafman]

...in a single, wouldn't V1 = Vr?

I'm thinking if the runway is short enough that you have to make a decision, you had better be airborn VERY shortly after that time...

-mini

This was a rhetorical question.