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The go/no go decision on takeoff for airliners is calculated as a V1 speed. The question is, why is there no time or distance for V1 calculated. In other words, if we don't reach V1 in X seconds or by X point on the runway the takeoff is rejected.

An example of where this might have prevented disaster is Air Florida Flight 90. The engine inlet probes were frozen, resulting in an inaccurate EPR reading. This caused them to set thrust much lower than they should have. They reached V1 and VR but with degraded lift, due to icing, they were not able to climb over the bridge. The FO felt something was wrong, but the captain told him it was fine and called V1.

The chain of events leading to this accident was very long and full of bad decisions. But had there been a hard and fast rule that if V1 has not been reached by this point then reject takeoff, the chain could have been broken.

Another example of where degraded performance resulted in too long of a takeoff roll is the Lokomotiv YAK-42 crash, although it doesn't appear they even had a V1 speed calculated either. They weren't accelerating fast enough, but the captain carried on expecting it to lift off at any moment. They went right off the end of the runway before it did. The reduced acceleration was obviously not enough to make the captain reject right away and by the time it did become obvious he was out of options.

There may be an unknown number of runway excursions where there was degraded performance and the crew did reject takeoff, but too late to stop.

If an engine has a problem it may continue to function in a reduced capacity then finally failing after V1. The procedures call for continued takeoff after that point, but they may have used up too much runway already.

There are myriad things that can cause degraded performance on a takeoff roll. Anything from tires, brakes, engines to flap configuration, even runway condition can slow you down and it may not be obvious that it's occurring. Many of these things won't show up on an instrument scan or a checklist. It just seems like there's no specific guidance as to how long you wait for V1. How does a pilot know when it's time to call it off?

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    $\begingroup$ Look at takeoff performance monitoring systems. $\endgroup$
    – fooot
    Aug 22, 2017 at 22:03
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    $\begingroup$ @mins That's pretty much the gist of it, yeah. It's a long question already. Trying to keep it simple $\endgroup$
    – TomMcW
    Aug 22, 2017 at 23:54
  • $\begingroup$ There is a recent Aviation Week article (aviationweek.com/business-aviation/…) discussing the danger of RTOs at high speed, and some of the comments on that story make the same observation about the use of only V1. $\endgroup$
    – Penguin
    Aug 24, 2017 at 10:20

2 Answers 2

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Something like that has been desired since at least the mid-80s; the issue is at least twofold: the system (whether manual or automated), and the use thereof (human factors).

Side note: it's different for a military jet trainer with higher thrust-to-weight ratio and relatively consistent takeoff mass (see this other answer).

For transport planes, V1 is not as simple as it takes time t or distance d to reach. The variables are a lot, and a pre-takeoff calculation may not match the exact conditions during the actual takeoff, meaning an error band needs to be applied – a few airplane lengths' worth:

  • Time: padding the time to avoid false positives defeats the purpose of a simple countdown timer.
  • Distance: unless the plane is nearing the end of the runway, gauging how much distance is remaining (assuming no low visibility), and comparing with a pre-takeoff figure, is not easy to handle without automation.

Enter: automatic performance monitoring. This has been a hurdle, in terms of the algorithm and processing power, until, academically speaking, the early 2000s. But, the human factors issue remains:


Training for and focusing on two things rather than one, and the added decision making and reaction time thereof, and the risk of false positive high speed rejections, are all big issues. Remember, V1 is not a speed at which a decision is made – the speed itself is the decision, even when it's about to be reached.

@fooot mentioned that Honeywell unveiled a system in 2014 that monitors takeoff performance. But, as of 2021, it's yet to be added to their Mark V-A EGPWS (the added processing power makes it possible). And when it does, it won't wait until it's near the V1 time/distance, rather it will call out a slow acceleration early on, to allow a slow speed RTO. This is easier to train for, and safer in terms of the speed at which a rejection would take place. It is also why, for example, the Boeing 787 inhibits the low-thrust warning a complete six seconds prior to V1; high speeds are not the time for troubleshooting.


References and further reading:

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    $\begingroup$ Sorry, I still do not understand this, so please correct me in my assumptions. V1 and required EPR are computed from the combination of runway (length, slope, altitude, wind speed), aircraft type and aircraft configuration (weight, CG balance, maybe thrust reverser availability or what). Therefore this expects that at the given EPR, you reach V1 at a specific point of the runway so that applying full brakes and 0 power stops the aircraft still on the runway. So as a part of the calculation, you know where on the runway you are supposed to reach V1. (cont'd) $\endgroup$
    – yo'
    Aug 23, 2017 at 7:33
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    $\begingroup$ (cont'd) and therefore to know in what time you should do so. And heck, this could be a semi-automated system: pilots put the estimated time for reaching from say 30 kn to V1 into some stopwatch, and the stopwatch starts once the aircraft is at 30 knots and says "SPEED TIME CHECK" or whatever when the aircraft should have reached V1. Then it's up to the pilots to say "damn we're way too slow, something's wrong". And as a last note: you say "TLAR should be enough"; well, obviously it wasn't, and it brought planes down, and AFAIK, aviation does not like the term "... should be ...". $\endgroup$
    – yo'
    Aug 23, 2017 at 7:37
  • $\begingroup$ @yo': Sorry for being late. I wasn't sure how best to address your question and then I forgot; I've now updated the answer to hopefully get the points across more clearly. $\endgroup$
    – ymb1
    Nov 24, 2021 at 15:52
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Some operations, including the USAF, use an acceleration check speed at a given distance (usually 1,000-2,000'). Data is derived from normal acceleration data gathered during performance testing. The normal acceleration check speed (NACS) check can then be reduced to a Minimum Acceleration Check Speed (MACS) based on a formula from the aircraft's tech order.

On the GA side, the POH for a multi-engine airplane will include an Accelerate-Stop chart (basically V1 calcs). The calculated V1 speed will be associated with a runway distance. This can be (but maybe isn't often) used to check normal acceleration to validate the TOLD Data

Easy answer - takeoff roll/speed checks other than V1 are used, and even V1 has a correlated distance in the takeoff data.

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    $\begingroup$ I've never heard of anyone mentioning a specific abort point other than V1 in a briefing. Granted, the only briefings I ever read are from accident cvr's, so maybe they do and I just don't know it. $\endgroup$
    – TomMcW
    Aug 22, 2017 at 22:35
  • $\begingroup$ V1 will be the verbally briefed abort point, especially in Part 135/121 ops. Military will brief MACS/NACS speed/distance as part of the told brief, depending on local standards. On the GA side, the POH for the specific airplane will indicate what airspeed was used to reach the calculated Accel/stop distance. It would likely be found on a redirection line that includes Gross Takeoff Weight $\endgroup$
    – ANDY-S
    Aug 22, 2017 at 23:53
  • $\begingroup$ Redirection line? Is that a line drawn diagonally on a graph/chart? Didn't know there was a name for that! $\endgroup$
    – TomMcW
    Aug 22, 2017 at 23:59
  • $\begingroup$ I haven't run across an official name for the lines, but i guess since we are all on the same page, it works! $\endgroup$
    – ANDY-S
    Aug 25, 2017 at 1:23
  • $\begingroup$ FWIW, in the Navy we always briefed a Line Speed Check, (NACS/MACS) max abort, (V1) and minimum go, (min speed at which you could accelerate to VR in the runway remaining). If Max Abort exceeded Min Go there was a no-mans land where you could neither take off, nor abort. Unless you dropped the tailhook... We also briefed single engine climb rates - initial, stores emergency jettisoned, and stores off/gear up. $\endgroup$ Nov 25, 2021 at 3:17

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