4
$\begingroup$

enter image description here
(B738 AFM, 0-2000 feet elevation, up to 27°C) Example of V1 increasing with weight.

This valid question came up the other day here in chat.

The heavier the aircraft, the harder it is to stop.

  1. So why isn't V1 reduced to allow for better stopping?
  2. How is the max weight / V1 determined for a specific runway?

V1 is the speed by which the decision is made to continue or reject the takeoff.

Improve this question
$\endgroup$
0

1 Answer 1

Reset to default
6
$\begingroup$

Other than being controlled by Take-Off Run/Distance Available (TORA/TODA) or Accelerate Stop Distance Available (ASDA), all field limits, maximum $V_1$ is also controlled by $V_R$ (Speed for rotation) and $V_{MBE}$ (Maximum brake energy speed, but let's ignore $V_{MBE}$ in this context). $V_1$ must not exceed either.

$V_R$ increases with increasing gross weight.

This means that at low gross weight, theoretically, if the aircraft had not yet been rotated past a $V_1$ limited by $V_R$, sufficient distance may still exist to stop the aircraft on the remaining ASDA.

At higher gross weights, the higher $V_R$ required might allow for a higher $V_1$. Therefore, in response to: "So why isn't V1 reduced to allow for better stopping?": The higher $V_1$ still provides adequate margin for stopping on the ASDA, but gives you the option to abort the take-off at a higher speed with the increased $V_1$.

Rejected take-off (RTO) is still initiated when the engine failure is recognized, so a failure before the "increased" $V_1$ will result in RTO at the time of recognition of the failure, and not at $V_1$.

If performance allows, I prefer $V_1$ = $V_R$, as opposed to say $V_1$ = $V_R$ - 20 kt, on a short, slippery runway, which would imply that given an engine failure past $V_1$ you'd have to keep accelerating the aircraft with asymmetric thrust, on said runway, close to $V_{MCG}$ (minimum control speed on ground, which can be the lower limit for $V_1$).

Improve this answer
$\endgroup$
6
  • $\begingroup$ Your explanation seems to assume that an increased V1 would not lead to an overrun even if a rejected takeoff is performed at the increased V1. But how do you guarantee the aircraft using that V1 would not overrun the runway, when there is no data given on the length of the runway being used? $\endgroup$
    – lemonincider
    Commented Feb 14, 2018 at 14:48
  • 1
    $\begingroup$ @lemonincider I am not suggesting you increase your V1 arbitrarily. Depending on the what performance data have available, you can have a range of possible V1's. The lowest V1 (V1go) would be limited by minimum control speed on ground (Vmcg), and your single engine take-off performance vs TORA/TODA, and the highest V1 (V1stop) would be limited by your Accelerate-Stop Distance Required (ASDR) vs ASDA, Vr and Vmbe. A V1 that would make you overrun on RTO would be in excess of V1stop. $\endgroup$
    – Waked
    Commented Feb 14, 2018 at 16:19
  • 1
    $\begingroup$ @lemonincider (contd) Now the question becomes: How does the table give me a V1 without information about TORA/TODA/ASDA? The answer is that before using the table, you will need to determine Maximum Take-Off Weight (MTOW) in another table/model. For this, knowledge about TORA/TODA/ASDA (and other factors, such as wind, pressure altitude, temperature etc). is required. Your MTOW will be limited by (among other factors) ASDR vs ASDA, meaning that if your actual Take-Off Weight is below your calculated MTOW, the V1 in the above table will also be within V1go..V1stop $\endgroup$
    – Waked
    Commented Feb 14, 2018 at 16:21
  • $\begingroup$ Thank you so much! Your explanation clears many of my questions on this. So the Maximum Takeoff Weight must be determined using separate charts prior to deciding a V1 based on other factors, and as long as the actual takeoff weight stays below the MTOW, any V1s in the table guarantees a rejected takeoff not leading to a overrun! $\endgroup$
    – lemonincider
    Commented Feb 14, 2018 at 17:31
  • 1
    $\begingroup$ @lemonincider In a table like this, your V1/Vr/V2 is dictated by your actual weight, but as long as your actual weight does not exceed MTOW, the V1/Vr/V2 for your actual weight is ok. Please note that every performance manual will contain specific instructions, and that I'm only providing information on what is "typical". The manual will also contain instructions for adjustments for slippery runway, inoperative equipment etc. Although terminology for MTOW's differ, I am referring to Performance Limited MTOW (determined on the day), not the Structurally Limited MTOW (which is static). $\endgroup$
    – Waked
    Commented Feb 14, 2018 at 23:00