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According to this comment on my previous question about automatic instrument takeoffs (or, rather, the lack thereof), the reason takeoffs are always flown manually is to allow the pilot to quickly reject the takeoff if necessary:

A failure, like a tire blowing or engine fire requires pilot intervention immediately. It would be better if the pilot as at the controls when it happened rather than having to disengage the automatic systems and play "catch up" with flying the plane. I'm sure this will get there at some point, but right now it's just better to have your hands on the controls and mind in the game during this critical phase.

What I don’t get is why a takeoff would have to be manually rejected - an autopilot can react far more quickly than any human pilot could ever hope to do; applying the brakes, deploying the speedbrakes, and idling and then reversing the engines are all actions that could easily be automated (and, in fact, the first two already are, not least for this very reason); and, of the many reasons for rejecting a takeoff, all suggest avenues for automation.

For instance:

  • Engine failure - Have the autopilot continuously monitor engine parameters, and do an RTO if one engine fails prior to V1, or if multiple engines fail at any point during takeoff.
  • Flight control failure - Have the autopilot continuously compare commanded to actual control surface position, and do an RTO if, at any point during the takeoff, one or more control surfaces fails to move when commanded, moves an insufficient amount, or moves in the wrong direction, or if one or more control surfaces moves when not commanded. Also have the autopilot continuously monitor pressure in all hydraulic systems (for aircraft so equipped), and do an RTO if a partial hydraulic failure (or, for aircraft with manual-reversion capability, even a total hydraulic failure) occurs prior to V1, or if an aircraft without manual-reversion capability suffers a total hydraulic failure at any point during the takeoff.
  • Pitot-static failure - Have the autopilot continuously monitor the altimeter and airspeed indicator readings, and do an RTO if the altimeter readings disagree (within a certain tolerance) with each other and/or with the runway elevation (for instance, if your altimeter says your altitude is 3 kft MSL on the runway at Amsterdam Schiphol, or reads 0 ft MSL in Mexico City, either someone royally screwed up the altimeter setting, or your pitot-static system is malfunctioning; either way, an RTO is warranted), if the airspeed indicators disagree with each other (again, within a certain tolerance), or if one or both airspeed indicators fail to come alive within a reasonable time.
  • Et cetera - Et cetera, et cetera.

None of this would require the use of any instrumentation not already in place, little if any modification of the autopilot (other than the software update necessary to program the rejected-takeoff routines into it), or of what it can control, would be necessary, and automating rejected takeoffs would considerably increase operational capabilities (since, firstly, the quicker reaction time of an autopilot versus a human pilot would decrease the time needed to initiate a rejected takeoff, thus shortening the timespan from when the rejected takeoff is commanded until the aircraft comes to a halt, thus shortening the distance needed for a rejected takeoff, thus shortening the runway length required for a given aircraft at a given weight with given atmospheric conditions, thus allowing the aircraft to operate from shorter runways, and/or from runways at higher altitudes, and/or at heavier weights, and/or during hotter and/or humider weather; and, secondly, because adding autoRTO capability would remove the final obstacle to automated instrument takeoffs, which would allow safe takeoffs in much lower, or even zero, visibility, which would - in the same vein as ILS-guided autolanding - greatly ease foul-weather operations) and safety (due, firstly, to the aforementioned decrease in the amount of runway needed for an RTO, and, secondly, to the ability to reduce or eliminate the large number of crashes caused, for instance, by pilots rejecting at high speeds for blown tyres) - so why can’t or don’t aircraft manufacturers build autoRTO capability into their product offerings?

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  • $\begingroup$ What if one of those automated systems fails during an auto-takeoff and the pilot hasn't hand-flown a take off in a year, or a month, or even a week? Piloting is a perishable skill (much like driving) and takes regular practice to keep the skills fresh. That's why pilots don't fly auto-land every time - the systems aren't available everywhere, they can fail, and when they do, the pilot needs to be 100% on his game. $\endgroup$ – FreeMan Mar 15 at 12:06
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What about a deer on the runway?

What about a BANG coming from the back somewhere?

What about a bird strike?

What about some indistinct object down the way that looks like it could be an airplane encroaching on the runway?

What about a WARNING light coming on?

What about a weird vibration starting?

What about an engine starting to stall/surge (whoomp whoomp whoomp) some time in the roll?

Rejects, especially high energy ones above the high energy threshold speed (it's why there is a speed call usually 80 or 90 kt) are considered high risk activities and often involve snap judgement calls. You will reject with less of a prompting if you are still in the low energy phase.

There are times when a pilot will reject even though he's just passing V1, if something happens that leaves doubt as to flyability and the runway is longer than necessary so there is still margin to stop. You have a second to decide.

It's not something you can automate. Way too many variables.

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    $\begingroup$ Yep. A tire blowing is another judgement call that varies with circumstances. If the runway is barely long enough, and I'm just shy of V1, where a blown tire means a probable overrun, I might continue and takeoff and head over to an airport with a long runway to land on. If the runway has ample stop margin, I'd probably reject and take my chances with what I had. $\endgroup$ – John K Mar 15 at 4:41
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    $\begingroup$ And why should the pilot trust an automated rejection system if there's already failures with the aircraft? Something's definitely not right with his machine, why trust any part of it, eh? $\endgroup$ – jwenting Mar 15 at 4:44
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    $\begingroup$ "just passing V1 [...] the runway is longer than necessary so there is still margin to stop" Isn't the airplane by definition then below V1? $\endgroup$ – a CVn Mar 15 at 10:54
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    $\begingroup$ Theoretically, the plane's systems would know about the WARNING light even before the bulb illuminated and would be able to react more quickly. However, if it's a warning, is it one worth an RTO, or is it one that we can continue with... $\endgroup$ – FreeMan Mar 15 at 12:07
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    $\begingroup$ "What about a warning light coming on?" is a bad example. Clearly the computer system that turned on the light could also initiate other action. Also, while you've convincingly argued that we can't automate all cases, you don't address the possibility of automating some cases. (Nonetheless, +1) $\endgroup$ – David Richerby Mar 15 at 17:04

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