V1 : Decision speed or Action initiation speed. All about V1

All about V1 :

A lot of people find it difficult to understand about V1 : whether its a decision speed or action initiation speed. Whats the significance of it ?

Lets put some light on the whole thing !! Lets see what the books say about V1 :

Definition : V1 is the speed from which an aeroplane can both

* Safely go with in the takeoff distance available.

* Safely stop with in the acceleration stop distance available.

In either cases aeroplane uses all the distance available.V1 is a speed for a particular Take Off Mass and therefore varies with change in TOM( Take off Mass).

For a Field Length Limited Take Off Mass (FLTOM)  there can only be one V1.

For any Take Off Mass lighter than FLTOM there is a range of speeds/V1 between Vstop and Vgo.

Lets define Vstop and Vgo before we proceed any further

Vstop : If rejecting a take off  Vstop is the fastest speed from which an aeroplane must be able to stop within the acceleration stop distance available.

larger the ASDA faster the Vstop but for a given ASDA it comes down to Take Off Mass.

Vstop faster for a light aeroplane than a heavier one. ( A lighter aeroplane accelerates more quickly and achieve faster speeds with in the same distance and also decelerate more rapidly after a decision to reject a takeoff)

Vgo : Vgo is the slowest speed from which a takeoff can be continued having just suffered an engine failure.

Larger the TODA slower the Vgo. Therefore for a given TODA Vgo varies with TOM.

Higher the TOM higher the Vgo (Since light aeroplane accelerate quickly, the minimum speed to go is low when the aircraft mass is low and the aeroplane will be able to takeoff within the remaining TODA) 

Since   Vstop and Vgo vary with a lot of factors and are not generally available/mentioned in aircraft’s performance manual. Therefore it is best to go with V1 as it gives a safety margin too.

Go or Stop Decision :

A pilot may reject a takeoff  due to an engine or other major system failure but it is the engine failure situation that affects the ability to go due to due to large reduction in excess thrust. However both the engine failure and all engine operating (with other major system failure) situations must be allowed for in he stop case.

If the engine failure is recognised before V1, the takeoff must be rejected.

An engine failure is recognised after V1 means that the takoff must be continued and the pilot climbs and positions himself for an approach at the same airport, jettisoning fuel if practical and necessary or proceed to an alternate (whichever is appropriate in the situation)

Decision speed or Action Initiation speed :

Since different authorities define V1  differently :

FAA :

V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.

Some  Authorities  describes V1 as :

V₁ is the Decision speed (sometimes referred to as critical engine speed or critical engine failure speed) by which any decision to continue or reject  a takeoff must be made.

In the recent years there have been a lot of  incidents of rejected takeoffs and some of hem have had fatal results. And in all the incidents the non successful ones are generally the ones that were rejected near V1.

Therefore  the need of the hour is  a more broader, largely accepted and well defined V1.