I'm not debating that it is or isn't useful. Just because "it works" doesn't mean I can't be curious about why.
I'm wondering "WHERE DID IT COME FROM"? It's more than just an arbitrary number.
Given it's application across such a wide variety of aircraft -- and with the takeoff performance of those differing aircraft are also significantly different (depending on conditions, of course) -- why is that one number the same? Why has that number stayed the same despite changes in the way airspeed is delivered to displays in the cockpit (ergo, air data computers vs simple pitot-static systems)?
A similar question was asked on another forum and here were the answers:
1) "All these are indeed quite correct, but to answer the initial question:
Why 80 KTS, as it might have been chosen 70, or 90, 100, 85....
If I remember well my early aviation days, on a 707, there was no link between the rudder pedals and the nosewheel, as on later aircraft, and no tiller on the right side; during initial take off roll, and especially with cross wind, the captain had to keep his right hand on the thrust levers and his left hand on the tiller to keep the plane on the centerline using the tiller, while the FO would hold a forward pressure on the column, and this until the rudder became aerodynamically effective to be used for directioanal control on the runway. That speed was 80 Kts, which the FO would call so the captain could move his left hand from the tiller to the column. From there on "80Kts" became a "traditional" speed to call and to use for all the other purposes you all guys have mentioned"
2) Airbus seems to have 80kts (but sometimes 100kts) as their cross-check for this reason: The tech manual has a certain type of cautions/warnings that can come on in phase 4... which is 80kts to take off (possibly because take-off power must be acheived by then?) - basically seperates the high speed RTOs from the low speed RTOs.
3) A poster on another forum had this to say about Airbus crosscheck at 100kts:
The statistics and experience have shown that, as soon as the aircraft reaches 100 knots, the safest course of action is for the flight crew to continue the takeoff, unless a major failure or a serious situation occurs.
The takeoff roll is divided into a low and high speed segment. If the aircraft speed is less than 100 knots, the aircraft is considered to be in the low speed segment and an RTO decision leads to a low risk maneuver. If the aircraft speed is above 100 knots, the aircraft is considered to be in the high speed segment and an RTO decision may potentially involve more risks. The speed of 100 kt was chosen to help the captain make his/her decision and avoid unnecessary RTOs at high speeds.
www.skybrary.aero/bookshelf/books/197.pdf
That's all I could find....
