Great post, Old Smokey! I like the energy explanation leading up to V1. I also like the idea about safety margins applied to Vmo and I assume also Mmo. I’m printing this one and saving it. Also, John T, your post on V2 speed was excellent as well. I still have instructors telling me to slow down to V2 if they fail the engine just above V2. That one is getting printed as well.
If remember reading the Boeing/FAA Takeoff Safety Training Aid correctly, their recommendation was for the PNF/PM to call V1 speed at a point where the call would be completed at V1 speed. This was to preclude completion of the “V1” call above V1 speed. An event occurring simultaneously with the call could result in the initiation of the RTO above V1 speed. Since certification requirements, past and current, specifies a minimum 1 second delay between the engine failure and V1 (Vef ≤ 1 second ≤ V1) and with acceleration near V1 speed at approximately 3 to 6 knots per second, many operators have taken to initiating the call “V1”, so that the call is complete and the PF’s hands are off the thrust levers at V1 speed. This is important since V1 speed is the maximum speed the RTO can initiated, e.g., brakes applied, T/L to idle, etc, whatever is specified as the first step in the RTO procedure. I don’t believe the intent was to transition to the “GO Phase” 5 knots before V1 speed.
It’s interesting reading the different discussions on recognition time, reaction time, and time delays with regard to V1 speed. As I’ve tried to emphasize when I speak to people on this subject, the pilot needs to understand the certification basis applicable to their aircraft, which rule amendment is applicable. In the US, AC 25-7A, provides an excellent discussion on the certification rules, past and present, and how they are applied to V1/accelerate-stop determination.
Smokey, correctly me if I’m wrong, but in the US, there are three changes to FAR 25 that effect V1 speed and accelerate-stop determination (ref AC 25-7A):
FAR 25 – V1 = Critical Engine Failure Speed
V1 and Vmcg can be the same (Vmcg ≤ V1). Even though V1 speed is defined as “critical engine failure speed” this is a misnomer. In accelerate-stop distance determination, engine failure occurred 1 second prior to V1 speed (engine failure ≤ 1 second ≤ V1) even though Vef speed was not yet defined in the certification regulations.
After V1, a 1 second time delay was added between each step of the remaining RTO procedures (brakes ≤ 1 second ≤ T/L Idle ≤ 1 second ≤ Spoilers extend). If a command is required to actuate the deceleration device, then a 2 second time delay is applied instead of the 1-second delay. The time delay assumes no aircraft deceleration during application.
FAR 25-amd 48 (1978) – V1 = Takeoff Decision Speed
Three major changes occurred. First, Vef was established as critical engine failure speed with the requirement that Vmcg ≤ Vef. V1 speed was determined by adding the speed gained during the time interval (1 second minimum) between the instant the engine is fails and the instant at which the test pilot recognizes and reacts as indicated by the application of the first retarding means. Therefore, the distinction between engine failure (Vef) and first pilot RTO action (V1) was also established. V1 not only ends the decision process, it established the speed at which the pilot needs to perform the first action to stop the aircraft.
Second, a 2 second time delay is applied between V1 and the first stopping action during which the aircraft continues to accelerate with all engines operating at takeoff thrust. Additional time delays between device activations are not applied for the first 2 devices.
For any number of devices beyond 2, however, an additional 1-second should be included for each device.
Third, the requirement to determine all-engine accelerate-stop distance was added.
FAR 25-amd 92 (1998)
This change again revised the accelerate-stop criteria and added requirements for:
a) The stopping capability of the aircraft as effected by brake wear (fully worn limit of their allowable wear range)
b) Wet runway as well as dry runway A/S distance.
c) Removed reference to “Takeoff Decision Speed” from definition of V1 speed
d) Replaced the “2 seconds of continued acceleration beyond V1” from FAR 25-48 with a distance increment equivalent to 2 seconds at V1 speed.
e) A/S distance must account for the highest speed reached during the RTO maneuver including speeds higher than V1.
f) Permits credit for reverse thrust in determining wet runway A/S distance and explicitly deny use of reverse thrust credit for dry runway
g) Require a maximum brake energy A/S test with not more than 10% of the allowable brake wear range remaining on each brake assembly.
Accelerate-stop distance 2-second delay is only a method to calculate the required distance increment and is not part of the A/S braking sequence. AFM accelerate-stop performance data must account for any residual acceleration that occurs after V1 while the airplane and its systems are stabilized in the braking configuration.
Understanding the differences between and more importantly the limitations inherent with these certification rules is necessary for pilot to completely understand V1 & accelerate-stop distance.
Rejecting a takeoff above V1 has significant consequences, as does taking an aircraft into the air that’s not airworthy. Continuing a takeoff following an engine failure below Vef also has consequences. As pointed out, the screen height will be less than 35 ft (15 ft on wet runways, FAR 25-92 aircraft). As one who is now familiar with the growing problem of close-in obstacles, those that penetrate a 62.5:1 takeoff surface (1.6% NTOFP), there are many airports where lower screen heights pose a significant risk.
Just my two cents worth on the subject
Rich Boll
Wichita, KS
Last edited by richjb; 18th Feb 2006 at 21:58.