There have been many accidents from rotating too soon ... starting with two Comet 1s!

It was learned from the two Comet accidents that while the aircraft CAN rotate early, the huge amount of drag increase caused may prevent the aircraft from accelerating to a flying speed. That is why we fly jet transport aircraft the way we do.

Sure, if one's Vr was say 150 knots and V2 of say 155 knots, you actually could lift the nose off the ground at around 120 ... but then you'd never see 150, or flight.

These were very lucky people indeed. Their biggest help here was probably the very capable low speed characteristics of the early B737 wing. They'd never get away with that in an NG!

D L X wrote:

rcair1 wrote:

So - the wake turbulence of an aircraft ahead of you will be below it's flight path and will not start till THEY rotate.
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That means that, if your aircraft has the performance to do so, you can plan your takeoff to lift off before their lift off point and stay above their climb path.

How long does wake turbulence from a preceding aircraft last? (At least on or near the takeoff runway?)

Depends on the size of the aircraft and the wind. Wake turbulence primarily is due to the vortex off the wingtip as the higher pressure air below spills out and up at the tip. The reason it does not start till you rotate is because the pressure differential is not really there till the wing starts flying (rotation). I don't know why it tends to descent and move out - not an aeronautical engineer. So - in calm conditions, imagine 2 vortexes off the wings, rotating opposite directions, trending down and out from the flight path. Now, add in a bit of cross wind and the "upwind" one may move across the runway. It is pretty hard to predict, but I don't think there is a case where it goes up, which is why planning your flight path to be above the departing a/c is good.

As somebody stated, if you are not able to do that, wait a bit - 30 seconds, a minute or so. Not a long time.

Here is a pretty nice video showing it on a 340 landing in fog.

https://youtu.be/QZkggFzAtEc

longhauler wrote:

There have been many accidents from rotating too soon ... starting with two Comet 1s!

It was learned from the two Comet accidents that while the aircraft CAN rotate early, the huge amount of drag increase caused may prevent the aircraft from accelerating to a flying speed. That is why we fly jet transport aircraft the way we do.

Sure, if one's Vr was say 150 knots and V2 of say 155 knots, you actually could lift the nose off the ground at around 120 ... but then you'd never see 150, or flight.

These were very lucky people indeed. Their biggest help here was probably the very capable low speed characteristics of the early B737 wing. They'd never get away with that in an NG!

Thank you very much for your input as professional. Can you tell us which Comet accidents you are referring to?

longhauler wrote:

These were very lucky people indeed. Their biggest help here was probably the very capable low speed characteristics of the early B737 wing. They'd never get away with that in an NG!

This was an NG, wasn't it? A 737-700?

rcair1 wrote:

So - in calm conditions, imagine 2 vortexes off the wings, rotating opposite directions, trending down and out from the flight path.

Right, so in this case, the waking TK plane was actually crossing the path of the RAM taking off, about a kilometer down the runway. Those wake vortices are pushing away from that point. The left wake moving towards the RAM (causing a headwind?) and the right wake is moving in the same direction as the RAM, causing a tailwind should the RAM run into it. If this change of direction happened near the point that the RAM rotated, could that have caused the plane's air velocity to suddenly move from above Vr to below Vr?

D L X wrote:

rcair1 wrote:

So - in calm conditions, imagine 2 vortexes off the wings, rotating opposite directions, trending down and out from the flight path.

Right, so in this case, the waking TK plane was actually crossing the path of the RAM taking off, about a kilometer down the runway. Those wake vortices are pushing away from that point. The left wake moving towards the RAM (causing a headwind?) and the right wake is moving in the same direction as the RAM, causing a tailwind should the RAM run into it. If this change of direction happened near the point that the RAM rotated, could that have caused the plane's air velocity to suddenly move from above Vr to below Vr?

I've never heard of that - but it does not sound unreasonable. I've not looked at the geometry of the runway's and takeoffs well enough to comment really.

D L X wrote:

This was an NG, wasn't it? A 737-700?

My mistake.
You are absolutely right. They are very lucky indeed.

N14AZ wrote:

Thank you very much for your input as professional. Can you tell us which Comet accidents you are referring to?

BOAC Comet 1, G-ALYZ, at Rome October 26, 1952.
Canadian Pacific Comet 1A, CF-CUN, at Karachi March 2, 1953.

Both of these accidents were initially deemed "pilot error". Although after investigation of the later CP accident the "gotcha" was made apparent and now was avoidable.

VirginFlyer wrote:

With higher flap settings, you can become airborne at a lower speed, but you also reduce your angle of climb, so if the goal is to clear an obstacle (or in this case wake turbulence), climbing out with a higher flap setting can be counterproductive.

V/F

Interesting, and that makes sense. It also adds fuel to the avoiding wake turbulence theory since the RAM appears to have used a Flaps 1 takeoff.

Looking at the video again there seems to be a lot of wing flex towards the beginning of the take off run so maybe they where affected by windshield or some wake turbulence down the runway.