@LGB

Obviously the underlying physics are relatively simple, and one could derive a rule of thumb in a manner similar to that you describe. But a couple of points:

TORA is pretty much irrelevant as a number for this purpose, since my time to a given speed should be dependnet upon my thrust and weight, not upon how much runway is in front of me. A "required" rather than "available" distance would at least be related to the aircraft (but TODR isn't much more use, because ....)

Similarly, V2 is an in-air speed, and really doesn't help you out much; there's too much going on to vary the acceleration between V1 and V2 for V2 to be much use in figuring out ground accel numbers.

What you'd really want are distances and speeds for a defined, on-ground, point - either at V1 or Vr would be best; then you could put those in your formulae and simplify them if you wanted a simple rule of thumb. The problem, of course, is that OEMs don't routinely publish distances to intermediate speeds, so the data aren't readily available.

To be honest, if we were dealing only with operations at max thrusts it would be relatively trivial to add an axis to a V1 vs weight type AFM chart which also showed time, so that you could read off both V1 and time-to-V1 from the same chart. The problem would come when people flex, as the time will obviously vary. Maybe there could be some kind of factor based on the amount of flex, but now we've added a new item to calculate, which wasn't the idea.

FearlessFreep re your excellent point “ check the "acceleration rate" of the airspeed needle and the corresponding position on the runway”, did you mean checking the airspeed trend vector on the modern ASI?

If so, I agree that this is of value, but for reliable results some knowledge of the technicalities behind the display are required, i.e. if the display is only derived from ‘speed’ then the display is relative to the air-mass acceleration - which still is very important to getting airborne; this is not the same acceleration check as a display that uses an ‘inertial accel / speed mix’, although both are valid as a gross error check. Similar use can be made of HUD FPV acceleration cues where displayed.

The issue of reduced thrust is another opportunity for the pilot to make a judgement on the safety margins available; how many of us really consider ‘what if’ to decide not to use the reduced option, i.e. on a wet runway reduced thrust is not used if it's the regulation, but if not mandated, then what?

The failure to provide a technical solution to the acceleration problem (due to many variables and integrity issues) is similar to that of providing an alert before landing if the aircraft’s energy is too high for the runway length / conditions. Even with generous safety margins many overruns still occur; the numbers of events suggest that this is a greater problem than the take off case.

alf5071h

You bring up some interesting points. Where I am employed we are glass cockpit equipped - only in that there are little pieces of circular glass scattered all over the place. I fly 74 Classics if that gives you any insight.

The acceleration values that I would be interpreting would be I assume specifically based on as you put it airmass - acceleration. I doubt that the CADC's in our aircraft would be inputting any inertial data. I guess you could on the mighty Carousel IVA select TK/GS hit the hold button and watch the acceleration units, though quite frankly there is a bit too much other stuff going on during takeoff in the Classic. What I am looking for, and this might be hard for me to fully elucidate is a rate of change or for lack of a better term "airspeed needle acceleration" that would be commensurate to previously noted values. If at any point I see a notable lag I will quickly check the opposite side ASI to see if it is reading the same. All of this is while monitoring our relative runway position. During the takeoff run I am always endeavoring to constantly monitoring (of course this is tempered with fatigue constraints - I cannot guarantee that I will be at 100% efficiency 100% of the time) our engine indications, acceleration, airspeed and runway position. These are the only tools that I have to utilize at this point in time.

Thinking more of the timing values that people have used in the past I do not know how effective these would truly be. There are so many variables involved that could change the value by a number of seconds which might remove any safety factor that the timed value would represent. Some of these factors are; rolling takeoff or static takeoff, how quickly and efficiently takeoff power is set, engine age and efficiency, i.e., available rated thrust, delays in starting the timing and any of a number of other factors that could either add or subtract a few seconds. There might be a tendency that once people would see that the timing/acceleration values were met an assumption could be made that everything was ok. With this goal met there would be a possible lack of further vigilance during the takeoff run.

As far as selecting reduced over max thrust, standing water or clutter (as per definition) will negate it's use specifically by regulation (at least in our case). I will select max thrust dependent on a number of different variables. Operating out of PANC with low barometric pressure and close to max allowable takeoff weight usually is always enough to raise my eyebrows. I have found by experience that the runway analysis does not appear to truly represent actual performance in those conditions. If you have max thrust available you might as well use it. When you consider that we fly old aircraft with old engines I am going to be conservative in my utilization of reduced thrust under any conditions that I might construe as being a risk to safety.

Reading between the lines, the thread keeps coming back to a need for a real distance achieved. IMHO this should be second by second, not just at some arbitrary point like V1.

I have mentioned in the past, that I have had two occasions where a warning would have been of great benefit. Both occasions I was a young copilot and did not have the authority to abandon the takeoff. I think I mentioned that I thought that it was due to uneven slabs of concrete at the Northern Yugoslavian airfield and later a grossly overloaded aircraft. Both BAC 1-11s

This was a long time ago, and it was only two instances in 40 years, but on both occasions the captain took no action whatsoever. (At the time or later.) Both times I fire-walled the power and sat and waited...with other hand on the flap lever.

Perhaps it's the reason that I took a personal interest in performance computers.

It would seem to me that a takeoff performance computer function in the FMS could be adapted to read real-time input sensors like wheel speed, GPS, acceleration and ADCs to keep a running prediction of where on the runway the vital performance speeds of V1 and Vr will be achieved. When those points are projected to occur beyond the last points on the runway where either a stop or a go(as applicable) are still possible, a warning is generated. Rate of IAS increase to determine the go point and rate of groundspeed increase for the stop point. Wheel speed and inertial for data validation of GPS position. Might need faster processors.

As to how it may be integrated and how it will be used to make go/no go decisions during the takeoff roll.... Let the games begin!

Just my $0.02

Best regards,

Westhawk