V1
Sky job,
I’d be careful about unduly limiting V1 on non-dry runways. While reducing V1 does half reduce the AD; one should account for the increased time on those non-dry surfaces controlling the OEI case. Contaminated surfaces significantly complicate control by increasing the actual Vmcg.
I’d be careful about unduly limiting V1 on non-dry runways. While reducing V1 does half reduce the AD; one should account for the increased time on those non-dry surfaces controlling the OEI case. Contaminated surfaces significantly complicate control by increasing the actual Vmcg.
Some companies consider the risks of a limiting V1 stop to be higher than the risks of carrying a failure in to the air (I have not seen the stats to support this so make no comment on the validity), therefore they use the lowest V1 available.
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Not true
Providing that the AFM data is observed and the sums done correctly with respect to the AFM, the background certification stuff will be taken care of transparent to the user.
If VR is below V1
The AFM will never let you do that. Ergo, a mistake along the way.
However (and we don't do this for the obvious Standards reasons) providing the brake energy is OK and the ASDA adequate, the physics will permit such a non-SOP action.
When would you ever want a V1 which is 'artificially' lower than the highest V1 that you could safely use? In effect it commits you to 'go' when you could actually comfortably stop. When is that ever going to be advantageous?
If the risk assessment is such that one is more concerned about stopping than going. It is essential to keep firmly in mind the fact that the AFM and certification, generally, is based on probabilities, not guarantees.
It is held, generally, that a high speed abort, even with a generous ASDA pad, is a high stress, high risk business ...
Sometimes, the flight standards assessment is that a lower V1 might be a better option .. in general, more risk of killing yourself stopping than going.
Increasing (stop - my addition) safety margins...
Who says you can "comfortably" stop from V1?
Precisely.
Contaminated surfaces significantly complicate control by increasing the actual Vmcg.
How so ? Vmcg is based on nil nosewheel steering input and the numbers consider friction characteristics.. it may be prudent to consider a reduction in crosswind limits for tyre force considerations .. However, a low speed go decision can provide an interesting piloting experience.
On that point, crosswind from the "wrong" side will increase the real world, on the day, Vmcg.
therefore they use the lowest V1 available.
Choosing the lowest V1 might introduce some exciting handling in the go case .. very much needing sim exposure and practice to reduce the startle factor. However, somewhere in the middle range makes more sense than routinely going at max V1.
I recall a period instructing on a 732 sim which had had the rudder mod incorporated ... very much a handful for the pilot at critically low speeds .. but trainable with exposure. Just how valid the observations might have been for transfer to the real aircraft I don't know .. but the stick and rudder exposure certainly upped the ante regarding instrument scan rate in the continued takeoff case.
Providing that the AFM data is observed and the sums done correctly with respect to the AFM, the background certification stuff will be taken care of transparent to the user.
If VR is below V1
The AFM will never let you do that. Ergo, a mistake along the way.
However (and we don't do this for the obvious Standards reasons) providing the brake energy is OK and the ASDA adequate, the physics will permit such a non-SOP action.
When would you ever want a V1 which is 'artificially' lower than the highest V1 that you could safely use? In effect it commits you to 'go' when you could actually comfortably stop. When is that ever going to be advantageous?
If the risk assessment is such that one is more concerned about stopping than going. It is essential to keep firmly in mind the fact that the AFM and certification, generally, is based on probabilities, not guarantees.
It is held, generally, that a high speed abort, even with a generous ASDA pad, is a high stress, high risk business ...
Sometimes, the flight standards assessment is that a lower V1 might be a better option .. in general, more risk of killing yourself stopping than going.
Increasing (stop - my addition) safety margins...
Who says you can "comfortably" stop from V1?
Precisely.
Contaminated surfaces significantly complicate control by increasing the actual Vmcg.
How so ? Vmcg is based on nil nosewheel steering input and the numbers consider friction characteristics.. it may be prudent to consider a reduction in crosswind limits for tyre force considerations .. However, a low speed go decision can provide an interesting piloting experience.
On that point, crosswind from the "wrong" side will increase the real world, on the day, Vmcg.
therefore they use the lowest V1 available.
Choosing the lowest V1 might introduce some exciting handling in the go case .. very much needing sim exposure and practice to reduce the startle factor. However, somewhere in the middle range makes more sense than routinely going at max V1.
I recall a period instructing on a 732 sim which had had the rudder mod incorporated ... very much a handful for the pilot at critically low speeds .. but trainable with exposure. Just how valid the observations might have been for transfer to the real aircraft I don't know .. but the stick and rudder exposure certainly upped the ante regarding instrument scan rate in the continued takeoff case.
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A320 wrote, "Not true. V1 is the maximum speed at which it is safe to reject the takeoff. VR is the speed at which you take off"
Vr is a speed at which the rotation is commenced not the lift off speed. If one was to grab all the elevator available at Vr we would see a lot of sparks as it rolls down the runway. Now in a fairly straight wing aircraft not a problem, but with increase wing sweep and weight, save the tail skid and rotate reasonably and have V2 at left off.
I've seen it happen just to many times....
Vr is a speed at which the rotation is commenced not the lift off speed. If one was to grab all the elevator available at Vr we would see a lot of sparks as it rolls down the runway. Now in a fairly straight wing aircraft not a problem, but with increase wing sweep and weight, save the tail skid and rotate reasonably and have V2 at left off.
I've seen it happen just to many times....
Contaminated surfaces significantly complicate control by increasing the actual Vmcg.
How so ? Vmcg is based on nil nosewheel steering input and the numbers consider friction characteristics.. it may be prudent to consider a reduction in crosswind limits for tyre force considerations .. However, a low speed go decision can provide an interesting piloting experience.
How so ? Vmcg is based on nil nosewheel steering input and the numbers consider friction characteristics.. it may be prudent to consider a reduction in crosswind limits for tyre force considerations .. However, a low speed go decision can provide an interesting piloting experience.
You got me doubting my fading (only a tiny bit) memory, so I dusted off the C-5 performance manual which is where I remembered Vmcg being corrected for contamination as well as crosswind. Calculating Vmcg was done as part of each take-off, run by the F/E and checked by the captain. Now, it’s on calculators, then paper spaghetti charts.
I ran a few samples and my memory was correct, there were corrections for each. With Zero crosswind, 0C, these are my results for three conditions: dry Vmcg (69 KIAS) to wet Vmcg (87 KIAS) to compacted snow (97 KIAS). Yes, the chart is titled Vmcg, 3-engine, no nosewheel steering. I can only guess the reason is that while on the ground the forces opposing the yaw are applied thru the tires. I’ve never looked elsewhere or seen Vmcg charts on civil types.
BTW, the beast increased Vmcg for crosswind component almost exactly 1kt for 1kt.
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You got me doubting my fading (only a tiny bit) memory
Join the club, good sir.
I’ve never looked elsewhere or seen Vmcg charts on civil types.
It would, no doubt, be tied up with the C5's characteristics and certification .. both of which are outside my ken.
the beast increased Vmcg for crosswind component almost exactly 1kt for 1kt.
Thanks for that. My rule of thumb for 4-motor-jobs has been 1kt/kt or a bit more .. good to have more evidence.
Join the club, good sir.
I’ve never looked elsewhere or seen Vmcg charts on civil types.
It would, no doubt, be tied up with the C5's characteristics and certification .. both of which are outside my ken.
the beast increased Vmcg for crosswind component almost exactly 1kt for 1kt.
Thanks for that. My rule of thumb for 4-motor-jobs has been 1kt/kt or a bit more .. good to have more evidence.
If it matters, the contaminated correction was applied before the crosswind. No doubt, it’s specific to the Lockheed certification and test. It was flight test data. The only question I had and have is that the basic physics are the same—B737-200 or C-5 and everything in between; so it should only be the magnitude, not the direction, if I may say. A lot of the Lockheed and USAF procedure was very conservative.
Having that chart in mind at the time, I did a very wet runway, strong crosswind (20 knots, at least) take-off in a Challenger at Biggin Hill. Being Biggin Hill, it was downhill, too. After airborne for Geneva, I mentioned to the F/O that I’d love to have the Vmcg chart for that take-off. “Why?” “Well, let me explain.....”
Having that chart in mind at the time, I did a very wet runway, strong crosswind (20 knots, at least) take-off in a Challenger at Biggin Hill. Being Biggin Hill, it was downhill, too. After airborne for Geneva, I mentioned to the F/O that I’d love to have the Vmcg chart for that take-off. “Why?” “Well, let me explain.....”
I do buy the cliff-edge argument, but for any normal runway (talking about big airports) why does a crew choose to potentially require some
rather than: Idle, brakes, TR.
A 96,000lb Global at +45 deg, calm wind: Rolling t/o, rotate at VR, climb to 50ft, idle, land ahead, stop. All on a 15,000ft runway! Okay, in the sim, but why would I ever have used a V1 < VR?
exciting handling in the go case .. very much needing sim exposure and practice to reduce the startle factor.
A 96,000lb Global at +45 deg, calm wind: Rolling t/o, rotate at VR, climb to 50ft, idle, land ahead, stop. All on a 15,000ft runway! Okay, in the sim, but why would I ever have used a V1 < VR?
Last edited by H Peacock; 26th Jan 2018 at 22:57.
It is not necessarily a crew decision. In some cases it may just be how the performance program spits out the data. Where I work we still use tabulated charts and have some limited visibility of the possible range of V1s. The V1 for our actual weight would be the lowest and the lower of Vr for actual weight or the V1 for the RTOW would be the highest available. Current procedure is to use the V1 for our actual weight, i.e. we are using the lowest available from the chart (we are "go minded"). In days past we would have used V1=Vr unless the RTOW V1 was lower (this equates to the highest V1 available - "stop minded").
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It's statistically safer to go than to attempt a stop with little margin. Hence a bias which makes going more likely increases safety.
The exception is when the terrain is challenging.
It's statistically safer to go than to attempt a stop with little margin.
Ah, Boeing?
Aero 11 - Rejected Takeoff Studies
And yes, usually there is some crew screw up involved. Think about it. Flight test crews head out to the airport knowing they are going to do rejected TOS. Crews headed for recurrent training know it's likely an engine will fail. Yet most high speed rejects aren't engine issues. On the line crews may repeat the standard mantra "I will reject for any master caution or master warning, an engine failure or ..." And yes, there's 2 seconds added for crew response time. History shows that may not be enough.
Aero 11 - Rejected Takeoff Studies
And yes, usually there is some crew screw up involved. Think about it. Flight test crews head out to the airport knowing they are going to do rejected TOS. Crews headed for recurrent training know it's likely an engine will fail. Yet most high speed rejects aren't engine issues. On the line crews may repeat the standard mantra "I will reject for any master caution or master warning, an engine failure or ..." And yes, there's 2 seconds added for crew response time. History shows that may not be enough.
Moderator
the basic physics are the same—B737-200 or C-5 and everything in between
Would love to hear anything further if you come across any detail ..
“Why?” “Well, let me explain.....”
Bit of a worry .. That's my main interest in Tech Log - we have an opportunity to discuss things which the minimalist training styles these days don't look at .. or, perhaps, don't have a clue.
Okay, in the sim, but why would I ever have used a V1 < VR?
Oftentimes, the numbers won't let you push V1 up to a comfortable value.
Consider the case of a min weight, min speed schedule with V1 down near Vmcg, decent crosswind .. and a critical failure. If the wind is from the wrong side, you're gone, no ifs, buts or maybes ... and it all happens super fast. In such a case, the pilot is faced with recognising that control has been lost and changing to an overspeed abort to salvage whatever might be salvageable.
The message we'd like to get across is this: if you are down near Vmcg with a strong crosswind, do have a think about pushing the V1 up (within the book range of figures) to give you a pad for the crosswind-driven increase in real world Vmcg. Alternatively, if you have the option, perhaps delay until the wind abates ?
The exception is when the terrain is challenging.
Even then, the gross/net fudge makes the continued takeoff preferable with typical terrain problems.
.. or the speed schedule is on minimum numbers and there is a stiff crosswind.
And yes, there's 2 seconds added for crew response time.
We need to keep in mind that the 2 second pad only applies to the more recent certifications. Older Types basically are raw data.
Would love to hear anything further if you come across any detail ..
“Why?” “Well, let me explain.....”
Bit of a worry .. That's my main interest in Tech Log - we have an opportunity to discuss things which the minimalist training styles these days don't look at .. or, perhaps, don't have a clue.
Okay, in the sim, but why would I ever have used a V1 < VR?
Oftentimes, the numbers won't let you push V1 up to a comfortable value.
Consider the case of a min weight, min speed schedule with V1 down near Vmcg, decent crosswind .. and a critical failure. If the wind is from the wrong side, you're gone, no ifs, buts or maybes ... and it all happens super fast. In such a case, the pilot is faced with recognising that control has been lost and changing to an overspeed abort to salvage whatever might be salvageable.
The message we'd like to get across is this: if you are down near Vmcg with a strong crosswind, do have a think about pushing the V1 up (within the book range of figures) to give you a pad for the crosswind-driven increase in real world Vmcg. Alternatively, if you have the option, perhaps delay until the wind abates ?
The exception is when the terrain is challenging.
Even then, the gross/net fudge makes the continued takeoff preferable with typical terrain problems.
.. or the speed schedule is on minimum numbers and there is a stiff crosswind.
And yes, there's 2 seconds added for crew response time.
We need to keep in mind that the 2 second pad only applies to the more recent certifications. Older Types basically are raw data.
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Muddy waters ahead.
Several have commented about V1 being a safe reject speed and what happen if during the reject. Some of this depends on certification. Not all agencies use reverse thrust in the calculation, just idle thrust and brakes. (Reverse thrust on a twin engine jet, with only one engine in maximum reverse and be a hand full and best. On a four engine maybe a little easier but not a lot of fun.)
Now if you happen to be at KMIA using a 3700 meter runway with an aircraft being rather light, no payload and maybe two hours of fuel, V1 could well exceed VR/V2. But, no pilot in his/her right mind would put V1 above VR/V2. So V1/VR are equal.
Many moons ago, at least several decades, a passenger aircraft departed San Juan, PR and aborted after V1 and at least after VR and maybe V2. The reject was not very successful, well into the grass and maybe a lagoon or two. So the lesson is reject at or below V1, if at V1 or above continue. It is generally recognized that it takes +/- three seconds to recognize, make a decision and react to the event. Thus, many experienced crew brief that approaching V1 we will continue.
It us usually better to get the airframe airborne, handle the problem, and return for an landing. Thus the airframe can be recycled to fly again than for scrap.
Several have commented about V1 being a safe reject speed and what happen if during the reject. Some of this depends on certification. Not all agencies use reverse thrust in the calculation, just idle thrust and brakes. (Reverse thrust on a twin engine jet, with only one engine in maximum reverse and be a hand full and best. On a four engine maybe a little easier but not a lot of fun.)
Now if you happen to be at KMIA using a 3700 meter runway with an aircraft being rather light, no payload and maybe two hours of fuel, V1 could well exceed VR/V2. But, no pilot in his/her right mind would put V1 above VR/V2. So V1/VR are equal.
Many moons ago, at least several decades, a passenger aircraft departed San Juan, PR and aborted after V1 and at least after VR and maybe V2. The reject was not very successful, well into the grass and maybe a lagoon or two. So the lesson is reject at or below V1, if at V1 or above continue. It is generally recognized that it takes +/- three seconds to recognize, make a decision and react to the event. Thus, many experienced crew brief that approaching V1 we will continue.
It us usually better to get the airframe airborne, handle the problem, and return for an landing. Thus the airframe can be recycled to fly again than for scrap.
V1 is not refusal speed, aka accelerate-stop, the first action of stopping is to be actioned at V1. The USAF always calculates refusal speed and a V1, which is handled the same way as civilians do. Yes, refusal speed could be the same as tire limit in many take-offs.
