Dank runway embarkation meander

[hawk37]

Scenario: The wind is 90 degrees off runway heading for take off. The pilot is passed a wind reading substantially less than the actual wind value. The runway is wet. The pilot completes a take off, does not abort.

Question1. Would a reasonably capable pilot with appropriate flight control inputs for cross wind technique find himself drifting upwind or downwind on the takeoff roll. Referring to the displacement from the centreline, not the aircraft's heading.

Question2. Would the depth of water affect the direction of the drift?

Consider that the aircraft may weathercock and/or experience downwind drift depending on the runway friction.

And no, I'm not looking for recommendations to follow SOP's, decline takeoff, or abort. All of these of course could go without saying. In this (perhaps) hypothetical situation, the aircraft does not depart the side of the runway, but continues the take off and gets airborne.

I would be interested in hearing opinions on the answers to Ques 1 and Ques 2.

[HercDriver]

I will answer #1...hopefully neither. the passed wind reading is just that ...passed....and for that matter always changing. The learned aviator in this case will put an appropriate correction in as he begins rolling and if he is worth his salt will be constantly making corrections to those inputs during the entire take off roll to keep the aircraft on centerline.... this of course is an ideal case.

[Bullethead]

Agree with HercDriver on Q1.

Question2. Would the depth of water affect the direction of the drift?


Drift is always going to be downwind, the depth of water on a runway is going to determine whether or not you attempt a take off at all.

Regards,
BH.

[hawk37]

"hopefully neither" Well, lets just say the magnitude of the crosswind was even more. Same wet runway. It is given that the aircraft does take off all engines, within the confines of the runway. The runway is "very very" wide, say 500 feet. You get my point. Wind from the left:

possibility one: The plane weather cocks left into the wind, and moves to the left of the centreline, the application of full RIGHT rudder is not enough to keep the aircraft on the centreline, and the aircraft moves UPWIND by, say, 30 feet from the centreline before recovering back to the centreline.

possibility two: Due to the runway being wet, and the tires not gripping as well as on a dry runway, the aircraft drifts downwind despite the application of full LEFT rudder opposite to the direction of drift, ie opposite to the application in possibility one above. The aircraft moves DOWNWIND by, say 30 feet from the centreline before recovering back to the centreline.

Is one possibility more likely than the other? In other words, which way does the aircraft go once the crosswind reaches a limit that the aircraft cannot fully control?

[hawk37]

Herc driver, you mention in your post "keep the aircraft on centerline.... this of course is an ideal case."

What would be the result in the non ideal case, even more crosswind that you can't hold the centreline? Say the runway is damp, but not that wet. Can you expect to drift downwind, or find the weather cocking causes the aircraft to track upwind?

[PEI_3721]

hawk37, interesting … first thoughts suggest that the aircraft will track downwind, which I think any type will do beyond any limiting condition. However, at or before that point there may be a ‘dynamic’ transient where depending on the aircraft characteristics it could deviate either side of the centre line.

Ref “crosswind landings” (http://www.flightsafety.org/alar/alar_bn8-7-crosswind.pdf) *. In fig 4, for the TO case, change the wheel braking force to thrust (opposite direction), but still aim to balance the forces.
With an increasing crosswind, the aircraft’s tendency to yaw probably depends on the forward speed, but it is the total side force which has to be considered; thus, it is more likely that the aircraft moves downwind when the cornering force is insufficient to maintain the centre line (fails to balance the crosswind force). During TO there could be a small lateral component from thrust if the aircraft yaws into wind; similarly aileron into wind – this would help.

In damp / wet conditions, the cornering force is reduced (reduced ‘lateral’ friction) thus, the crosswind component required to unbalance the aircraft's sideways movement is less.
Note that changes in friction can occur without a damp / wet surface. The runway surface materials / tyre condition can affect the side force. Thus, always respect the limiting value of crosswind and assess its relevance to the runway condition (physical and meteorological), any wind gusts, and the pilot’s ability / currency (capability).
Flight operations, like questions such as this, are rarely simple. It’s often the assumptions made in identifying the situation which cause the problems.

* Also available from Airbus - Corporate Information - Ethics & Commitments - Safety first library (http://www.airbus.com/en/corporate/ethics/safety_lib/index.html) - under Landing Techniques.

Other refs:
http://www.flightsafety.org/ppt/managing_threat.ppt
http://www.nlr-atsi.nl/downloads/NLR-TP-2006-324.pdf
http://www.nlr-atsi.com/downloads/NLR-TP-2001-217.pdf

[hawk37]

Thanks, PEI, very interesting info. I did read almost all of it. All related though to landing info, however the force diagrams can be applicable to take off cases, with changes for thrust and brakes, as you pointed out.
Reference figure 4, you DID mention it for the take off example, the overall longitudinal force actually provides a vector to prevent drift, whereas in the landing situation, the brakes actually add to the drift.

So...here's what I've come up with after thinking this over a few days, for the case of a crosswind that is just too much to keep the aircraft on the centreline:

For the situation of good runway mu value (dry/damp), a substantial crosswind will/may weathercock the aircraft into wind, and the aircraft will depart the centreline in the upwind direction the 30 feet given in my example, before the flight controls are effective enough.

For the situation of a poor runway mu value (standing water), a substantial crosswind will/may cause the aircraft to drift downwind 30 feet before the flight controls are effective enough.

This make sense?

What dept of water affects the direction of the excursion? Guess that would depend on a few further factors.

[Nightrider]

Hawk, shortly after wet there is the situation of contaminated.
If you "float" on the water during take-off, the runway must be considered contaminated....
In a contaminated situation your last assumption sounds pretty much realistic.

[Sir George Cayley]

The runway is wet. How do you know? Because you are told so. How does the ATCO know to tell you? Because an Ops person has told him. How does the Ops person know? Because the ATCO gave him permission to drive onto the runway to have a look. How long ago was it since the Ops person looked at the runway surface condition and how long after that did it take to pass the info to ATC? Once ATC had the info, how long after that was it that ATC gave you this info. What happened during this time period to the wind and the rain and the level of water on the runway?

Are you getting the thought that the decisions you make about take-off and or landing are not based on factual up to date info? Good:ok:

If the RWY is really just WET WET WET, then your departure or landing should not be a problem. Assuming that the equipment that measures the instantaneous wind, last highest gust and trend are accurately calibrated you will have no problem.:)

Think Swiss cheese, think James Reason and the ask yourself "How did I get here" Hmmn, Not so simple eh?

Sir George Cayley