Old Jan 19, 15, 12:15 pm
  #13  
chiph
 
Join Date: Jun 2005
Location: SFO
Programs: UA 1K/*G
Posts: 276
As a layperson it's difficult for me to understand that on the ground, aircraft have a critical "V1" speed below which it's safe to abort a takeoff, and after which you should always continue takeoff, even if there is an engine failure. I do know that the scary part of a takeoff is near the middle to end of the runway as you approach that speed and the pilot's options dwindle, but conceptually the way it works has some gaps for me…

I know the calculation of V1 speed is done on a per-equipment basis and considers things like "above this speed, it will not be possible to stop the plane because brakes aren't designed to do that much work", but it seems weird that this calculation doesn't include runway length or takeoff weight.

Instead, in what seems weird to me, "minimum required runway length" is calculated as a function of aircraft weight and V1 — so the a/c's rated maximum refusal speed determines how much room it needs, and not the other way around.

Why not say "oh boy, we've got 6 more miles of runway in front of us, even though my V1 is 124 mph and I'm going 126 mph, I can probably safely decelerate in the space available?"

Maybe a question for the 'ask a pilot' thread. Nice to hear that in this case, the pilot safely made the right call during an important phase of flight.
Originally Posted by PropClear View Post
Then you're having to rely on the crew knowing what their V1 is on every different runway (plus weather conditions, altitude, etc). Prolly a lot safer giving them one number, knowing that while scary, taking off above that speed has been tested extensively and can get them back around to land.
The V speeds (e.g., V1, V2, Vr, Vref, etc) are calculated per-takeoff by the flight management system (FMS) and takes into consideration the runway (including direction), winds, temperature, conditions (e.g., wet/dry), weight, etc.

The crew has the #s before takeoff and they're also displayed on the airspeed tape/display.

Also see http://en.wikipedia.org/wiki/Balanced_field_takeoff
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