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.
- 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?