Why is the takeoff decision point based only on speed and not time or distance?

Question

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 V_R 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?

Answer 1

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:

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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.

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References and further reading:

• Srivatsan, Raghavachari, David R. Downing, and Wayne H. Bryant. "Development of a takeoff performance monitoring system." AIAA Guidance NAS 1.15: 89001 (1986).

• Middleton, David B., Raghavachari Srivatsan, and Lee H. Person Jr. "Flight test of takeoff performance monitoring system." (1994).

• Zammit-Mangion, David, and Martin Eshelby. "An operational evaluation of a take-off performance monitoring algorithm." ICAS2002 Congress. 2002. (PDF)

• Pilot Guide to Takeoff Safety. FAA. (PDF)

Answer 2

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.