V1 confusion

[MalteseJambo]

Just been going back over my ATPL notes and have become a bit confused regarding V1.

Bristol notes Performance 3.14 definition
'v1 means the maximum speed in the take off at which the pilot must take the first action (e.g. apply brakes) to stop the aeroplane.

However... Bristol notes performance 5.8

'A further 2 seconds is allowed between calling 'stop' at V1 and actually starting to stop the aeroplane.'

Bit surprised of such a clear contradiction but perhaps i am misinterpreting it.

Any help much appreciated and before anyone asks yes i have tried searching but nothing answering my question.

Cheers

[felixthecat]

From my understanding the decision has to have been made by V1 and the extra 2 secs are a buffer to allow for reactions to the decision. But the decision has to have been made….

[kaptn]

At V1 the pilot can rotate for take off...2 seconds if for safety I think...in case where you didn't reach the actual rotation speed while your speed indicator shows it....what I mean is that V1 is théoritically what you see in your IAS, but not what you're measuring...correct me if I'm wrong.

[felixthecat]

V1 is not the rotate speed VR still is even with engine failure, V1 is a decision speed. Rotating at V1 will seriously compromise your take off performance. You still have to accelerate to VR, and the aircraft is still capable of doing so according to the certification.

[mutt]

V1 started as a decision speed whereas a crew could make the decision to abort, but the industry has decided that in order to enhance safety it was more prudent to make V1 an ACTION speed. In order to allow for pilot reactions, and for the aircraft to transition into a stopping configuration, a buffer of a couple of seconds in built into the calculations. This buffer isnt there for your use, so if you hear the call V1..... then its too late to abort.

At V1 the pilot can rotate for take off. This is incorrect, while there may be occasions where V1 is the same as VR and you could rotate at V1...... generally this isnt the case.

[Denti]

This is incorrect, while there may be occasions where V1 is the same as VR and you could rotate at V1...... generally this isnt the case.


As always, that depends on type. Back on the 737 classics usually Vr and V1 were either the same or within 3kts which meant we only called rotate and no extra V1 call.

[Mad (Flt) Scientist]

To add to some comments above, not only is the "2 seconds" a purely performance accounting item, intended to build some conservatism into what would otherwise be a very "test pilot" like scenario, and therefore in NO WAY to be considered as available to any crew conducting an abort to give them time for a leisurely chat about what to do next, but it also is not accounted for in any "real" terms.

That is, it's an additional DISTANCE added to the calculations, equivalent to 2 seconds at V1. But, whether "stopping" or "going", no aircraft will spend 2 seconds magically stuck at V1. If you were to sit on your hands at V1 for 2 seconds, you'll find that you've travelled somewhat further than the assigned distance "margin", since you'll be accelerating during the 2 seconds. You'll also now find you're trying to stop from significantly higher speed than V1, which means that all your stopping distances are out to lunch as well.

I've not done the explicit sums, though I'm sure they have been done, but I suspect that the actual margin of decision making variability which the "2 seconds" actually represents - assuming that you were NOT KE limited at V1 - is something like 3/4 second. Which, given that reaction times of the order of 1 second are routinely required for certification purposes, means there isn't any real margin at all, and certainly nothing anyone should be expecting to rely upon.

[moggiee]

Bristol notes Performance 3.14 definition
'v1 means the maximum speed in the take off at which the pilot must take the first action (e.g. apply brakes) to stop the aeroplane.

Interesting - especially as the first action taken to stop the aeroplane is to retard the thrust/power/throttle levers. There's no point hitting the brakes against the take off thrust of the the remaining 3 RB211s on your 747-400!

V1 is a decision speed and is the highest speed at which you may take the decision to abandon the take off, and then initiate an immediate rejected take off procedure. The "2 seconds" is a bit of a red herring as any additional time/distance allowed above V1 is a buffer that allows for the time that it takes for the nerve signal to run from your brain, down your arm and retard the thrust levers. A simultaneous and similar signal should be running down your legs to initiate braking (that may not be all that is running down your legs, of course!).

I think I'm right in saying that the FAA and CAA/JAA had slightly different ways of calculating reaction time but that notwithstanding that, the decision is made at V1 and the actions are taken as quickly as humanly possible after that.

I've only ever had to do one RTO on a large jet but I am absolutely certain that my reaction time was well inside 2 seconds, but then I always assume/expect the worst and hope to be ready for it.

[madlandrover]

I always assume/expect the worst and hope to be ready for it.

Sadly it sometimes happens as well :rolleyes:!