Originally Posted by Old Smokey
J_T, Mutt,
I think that the scariest in recent history was the gentleman believing that V1 was an arbitrary figure derived from the WAT limit.
yawskidball, a BFL condition will exist entirely dependant upon the type of AFM data that you're using. Most use it, some don't, some use Clearway and/or unequal Stopway, some don't. I prefer to work with unbalanced data (basically because I'm a very unbalanced person), it gives greater flexibility, particularly when factors affecting Accelerate-Stop, but not affecting Accelerate-Go (such as Anti-Skid being Inop) exist. One then flies at the most limiting of the 2 conditions.
If obstacles have forced you to optimise the Obstacle Climb Limit against the Field length used, thus reducing the Effective Operational Length (EOL), and you're using Balanced Field data from the AFM, then the operation is still BFL, even though you're not availing yourself of the full runway length.
Obstacle clearance is considered to 1500 feet, or GREATER, if required to satisfy obstacles (That's a very short version of the total picture).
Regards,
Old Smokey (Taking holidays now to enable sensible discourse to resume on PPruNe)
.
Is V1 then not
arbitrary?
But I doubt WAT limit can be enough to determine V1...
After all, what I see of the definitions...
A "balanced field" seems to make no sense except for airplanes that have more than one engine and can safely climb with one of engines inoperative, but no other failures.
The balanced field means that:
- an aircraft accelerates with all engines functional
- at a certain point, 1 engine out of 2 goes out - but no other failures occur
- 2 seconds later the aircraft shuts down the good engine, but does not apply reverse thrust
- and then the aircraft stops precisely at the end of the balanced field
- ass-uming that the wind throughout the accelerate-stop sequence is precisely unchanging
- ass-uming an ass-umed total mass of the craft, including payload (not the real, physical weight! loaded planes are NOT weighed!, cannot be done at a gate subject to wind)
and the BFL is then the shortest field length where the aircraft also could unstick, reach certain arbitrary height (15 feet?) before the end of runway and continue to climb at a certain rather arbitrary speed (V2) and quite arbitrary gradient (2,4 %, 2,7 %, 3,0 %, ...)
Surely there are plenty of other details, but it seems that you are already riddled with rather arbitrary ass-umptions, some of which are enacted by law, though.
So, V1 is already arbitrary on the, itself arbitrary, balanced field. Now think of a field longer than arbitrary...
Depending on the OEI thrust and WAT, several planes are known to be able to take off with OEI from the start of roll. Does it mean V1 equals zero, or negative for the matter?
And on the other hand, with a short take-off/landing plane on a long runway, it seems possible to get airborne in a short stretch of runway, hop to 35 or 15 or something feet and proceed to land on the rest of the same runway. Does it mean V1>>V2?
I understand that the stopping distance is relatively well physically constrained for a given real groundspeed of plane, real unchanging braking conditions and real unchanging functionality of the brakes. With, say, 4000 feet of runway left, the V1 might make a difference between coming to a halt 100 feet short of the runway end or tipping over to a ravine 100 feet beyond.
But obviously, it makes all the difference where you are at "V1": whether it actually is 3000 feet left, or 5000 feet...
Oh, and another example. Suppose headwinds increase at some point in takeoff. Does it mean that you are suddenly beyond V1? But you still have a lot of runway ahead, and your Accelerate-Stop chances are actually slightly improved (more wind drag). And the opposite - headwind decreasing or tailwind gusting, stops you from reaching V1 as the airspeed does not increase as planned, AND Accelerate-Go gets much harder - yet Accelerate-Stop is not getting any easier, and delaying abort would surely mean disastrous overrun...