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Old 5th Feb 2012, 11:54   #1 (permalink)
 
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737 max crosswind take-off roll. Question on rudder

Question: During the take off roll until VR in a strong crosswind component, aircraft will tend to yaw into the wind due to `weather-cocking` effect. The pilot counteracts by using rudder pedals to maintain the centreline of the runway.

When applying rudder pedals to maintain the centre line, the rudder will also move and after a certain speed is the prime force used to keep the aircraft straight. But does the nosewheel steering which also operates via the rudder pedals to a limiting degree, also operate (in other words turn the nosewheel) in addition to the rudder itself?

Put another way, during the high speed portion of the take off roll in a strong crosswind, what force is used to hold the centreline? Is it mainly nosewheel operation actuated by the rudder pedals - or is it only the rudder which is also also operated by the pedals - or both together. Where a gust may require a sudden full rudder application - however momentarily - would that cause the nosewheel to also turn and scrub the tyres?

I suppose the same argument could be applied to continuing the take off roll after engine failure. Does instant corrective rudder pedal operation also cause the nosewheel to turn as well as the rudder to deflect?

The subject arose during a discussion on simulator fidelity tests. Immediately after airborne in one particular 737 simulator in a 35 knot crosswind from the right, the aircraft momentarily dips the left wing causing the aircraft to turn slightly left before the weather-cocking effect takes place and simultaneously causing the aircraft to momentarily yaw into the wind. This unusual series of events over 2-4 seconds adds to the task of tracking the centreline during initial climb.

Immediately after rotation and the main wheels leave the runway, and with a 35 knot crosswind from the right, in the real aircraft what happens? Does it yaw and roll slightly to the right caused by weathercocking. Or does it momentarily bank left before weathercocking (as shown by a yaw and roll) to the right?

I have extensive experience in the real aircraft and don't ever recall a momentary bank downwind followed by initial weather-cocking into wind.
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Old 5th Feb 2012, 12:18   #2 (permalink)
 
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Yes it happens, Pilots are reluctant to hold enough aileron into wind to keep the upwind wing down on the roll because of fears of spoiler drag. As soon as the wheels leave the ground, the upwind wing has far more lift than the other and will roll downwind, requiring suddenly far more into wind aileron. Then the weathercocking effect takes effect because as the wheels lift off, you are instantly sideslipping. On the 747, the effect can be startling- I've seen 90 degrees aileron instantaneously applied into wind to correct the situation. I've been watching and sometimes suggested in strong winds further aileron would be a good idea. When it's not, it's interesting to see the result! I even once witnessed a pilot who didn't apply any aileron on the roll in a fresh crosswind- I think he learnt the point quickly, but I wondered how the training section hadn't picked this up in the simulator.

The same effect seems less pronounced on the 737.

It sounds arrogant, but it's the one place I disagree with Boeing on crosswind take-off recommendations. Spoiler drag below about 120 knots is, I believe, irrelevant. The spoilers aren't even that good for slowing jets up below 250 kts, so partial displacement below 120 kts isn't going to amount to more than 'a hill of beans'! Partial spoiler drag on take-off has little effect. But the BFCTM says otherwise.
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Old 5th Feb 2012, 12:44   #3 (permalink)
 
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I think the nosewheel steering via the rudder pedals gives control to the pilot until the rudder becomes efficient at higher speeds(80kts+).
At these higher speeds applying force of the rudder pedals will provide most control via the rudder itself.so no need to worry about the gust destroying your nosewheel and tires.
By the time of rotation,ailerons into wind should compensate for the rudder input.,too much ailerons input will cause a drop of wing/too much rudder will cause weatherCocking.

If once airborne you get rid of the rudder too quickly,youll get a low wing,if you get rid of ailerons inputs too quick youll get weathercocking.
Decrabbing should be done simultaniously by reducing ailerons and rudder inputs.
Never happened to you in a strong crosswind?? A little wing drop?
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Old 6th Feb 2012, 07:57   #4 (permalink)
 
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Quote:
Spoiler drag below about 120 knots is, I believe, irrelevant.
An F/O on the 737-400 told me of his experience of being PNF during a take off where the crosswind component was around 15 knots. The captain (from the old school of captains from a particular former Australian legacy airline) pre-set almost half control wheel deflection at the start of the take off roll and held that deflection for the whole take off run.

Within around 10 knots of V1 the acceleration appeared to slow markedly - in fact so much that much more runway was eaten up than a normal take off at the same weight. The F/O was concerned that V1 was now invalid.

Spoiler drag below about 120 knots is, I believe, irrelevant.
This contradicts the FCTM advice that" reverse thrust and speedbrake drag are most effective at high speed" while at the same time the FCTM displays a diagram indicating that landing with speed brakes not extended increases the landing distance by 630-900 ft. As the average touchdown speed on the 737 is in the region of 120 to 145 knots then clearly spoiler drag is significant near typical rotation speeds and unnecessary control wheel deflection throughout the take off roll will extend take off roll.

.
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Old 6th Feb 2012, 08:23   #5 (permalink)
 
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Quote:
landing with speed brakes not extended increases the landing distance by 630-900 ft
On landing the speed brakes make the wheel brakes more effective. It´s not just the aerodynamic drag...
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Old 6th Feb 2012, 11:22   #6 (permalink)
 
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Quote:
An F/O on the 737-400 told me of his experience of being PNF during a take off where the crosswind component was around 15 knots. The captain (from the old school of captains from a particular former Australian legacy airline) pre-set almost half control wheel deflection at the start of the take off roll and held that deflection for the whole take off run.

Within around 10 knots of V1 the acceleration appeared to slow markedly
Especially with pilots 'impressions', you have to be very cautious. In a max weight 747-200 take-off from a hot and sultry BOM, I was very concerned how close the end of the RW at the V1 call it was so close. I reported that the performance was incorrect and no way could we have stopped with an engine out and on 3 reverses. The airline Performance analysis team re-examined it and concluded that there was still 100m or so in hand. i didn't believe it for a moment. A week or so after getting their response, exactly that happened, the same service lost and engine at V1, same conditions, rejected on 2 reverses (1 locked out) and still stopped with....100m in hand.

I'm afraid an F/Os assessment of acceleration during TO is just not reliable. The human body can feel good acceleration, but as for sensing degrees of graduated power acceleration, it doesn't even start. Wind effects can do funny things to the ASI as well and make it appear acceleration has stopped. I do believe partial spoiler on one side around 120kts really has no effect, but that's not what Boeing says. But- having half control wheel deflection in a strong crosswind (over 20-25kts) is a good idea. All the surprises I ever saw on lift off were from not enough, not too much. On 747 and 737, we practically used to work on about 1 division on the boss/5kts, with the knowledge when you lifted off, you would need more immediately after the wheels lift off. So question- is it better to have more close to VR than get airborne and have to go to 90 degrees as I have seen frequently and near full deflection is applied to control the wing drop? You might not believe it, but it happens- watch carefully next time the other pilot does a strong crosswind takeoff.
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Old 6th Feb 2012, 11:40   #7 (permalink)
 
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speed brakes > spoilers?

If you will pardon a quick query from a non pilot is it not the case that full speed brake on landing gives deployment (obviously on both wings) of more panels through a greater angle than deployment of full spoiler for roll control? So drag is (intentionally) much greater then?
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Old 6th Feb 2012, 12:19   #8 (permalink)
 
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That is correct.
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Old 6th Feb 2012, 17:02   #9 (permalink)
 
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The comment about wing drop after lift off towards the downwind wing is also exacerbated by holding the downwind rudder on too long. It is the case that at rotation a little more into wind aileron is required. FCTM says to remove the cross control once airborne. Personally, I find it very manageable to reduce the downwind rudder during rotation to be zero at lift off. This reduces the amount of extra aileron required and also allows the a/c to take up the correct drift heading immediately after liftoff. It's all very smooth.
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Old 6th Feb 2012, 19:24   #10 (permalink)
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Question from initial message:
"... after rotation ... main wheels leave the runway ... 35 knot crosswind ... what happens?..."
TeeEmm -- you likely already considered this AAR's X-wind analysis [mostly regarding the on-runway directional control]. Some human-machine analysis might be useful during the airborne phase S&C interactions,
AAR-10-04 PDF Section 2.2.1 pg41+:
"...oscillatory ... pedal inputs were similar to each other in shape and differed from the smaller, incremental adjustments the captain made earlier in the takeoff roll. The captain’s switch to unusually large inputs changed the dynamics of the situation ... made it more challenging for him to subsequently control the airplane’s heading and track the runway centerline. ... to avoid overshooting ... after each large right rudder pedal input, the captain had to compensate by relaxing the ... pedal more than he would have had to for a smaller rudder pedal advancement. Furthermore, because of slight delays in the effect each rudder pedal adjustment had on the airplane’s rate of heading change, the captain had to anticipate the effect of each adjustment ahead of time. This task was very difficult for the captain because of the highly variable and unpredictable nature of the crosswind gusts...."
From PDF AAR-10-04 pg55:
"... data obtained ... revealed that only 4 out of 250,327 Boeing 737-500 takeoffs reviewed occurred in crosswind components of 30 knots or greater during the 8 years preceding the accident ... 58 additional events occurred involving other airplanes in Continental’s fleet during the same period ...

... the collection of operational flight data by onboard flight data recording devices and the subsequent analysis of ... potential safety vulnerabilities.... operational flight data can be linked to related airport, runway, and/or weather information, the data generated through these safety programs could prove valuable ..."


[from AAR pg 54] "... a manufacturer’s demonstrated crosswind is based on the successful accomplishment of three takeoffs and landings by a highly skilled test pilot and reflects the wind conditions that were available to the manufacturer for testing during the certification process. ... evaluation of an airplane’s crosswind takeoff and landing performance (and perceived handling qualities) in very gusty wind conditions is not required by Federal regulations ..."
Re' directional control JUST AFTER LIFTOFF, yaw x roll interactions, that brings to mind all the engine-out training mishaps (training-in-aircraft), and all that investigative S&C analysis. One item found in the AAR70-11 is a flight-test report from a Convair test-pilot, A.P. Wilson, "A Pilot Reviews Swept-Wing Jet Transport Takeoff", on pgs 13+, NTSB/AAR-70/11

Last edited by IGh; 6th Feb 2012 at 19:40.
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Old 6th Feb 2012, 20:35   #11 (permalink)
 
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Quote:
But- having half control wheel deflection in a strong crosswind (over 20-25kts) is a good idea. All the surprises I ever saw on lift off were from not enough, not too much. On 747 and 737, we practically used to work on about 1 division on the boss/5kts, with the knowledge when you lifted off, you would need more immediately after the wheels lift off. So question- is it better to have more close to VR than get airborne and have to go to 90 degrees as I have seen frequently and near full deflection is applied to control the wing drop? You might not believe it, but it happens- watch carefully next time the other pilot does a strong crosswind takeoff.
Notso Fantastic, couldn't have put it better myself! I always say a bit too much aileron is better than not enough - I know the question of spoiler deflection is brought up but, like you, I think this is not a relevant factor. Control of the aircraft in terms of keeping the wings level on lift off is far more important. (I speak as one who flew the B707 in which it was critical due pod clearance when the nose was pitched up)
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Old 6th Feb 2012, 21:11   #12 (permalink)
 
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@Tee Emm

It is true that the FCTM says " reverse thrust and speedbrake drag are most effective at high speed ", but you get full ground and flight spoilers after touchdown.
For takeoff, while holding the wheel displaced you are not getting any ground spoiler deflection, hence any comparison with the landing distance penalty for not using speedbrakes is not appropriate, in my opinion.

I agree with rat5's techique, that is what I use too, seems to work fine.
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Old 6th Feb 2012, 23:08   #13 (permalink)
 
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As pilots we put in the appropriate aileron into the wind and judge what it takes to keep the wings level at rotation. We never let the upwind wing come up. It is quite simple to do.
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Old 7th Feb 2012, 06:52   #14 (permalink)
 
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Thank you all for your informative comments. It was greatly appreciated. Interestingly, looking through a 1976 B737-200 FCTM it mentions starting the crosswind take off run with "preset" aileron position then gradually changing the wheel position as speed increases. In later editions of the FCTM the reference to preset aileron has been removed. Similarly I distinctly recall seeing another 737FCTM that stated unnecessary large control wheel angles can result in a longer take off distance. That seems to have since been removed, too
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Old 7th Feb 2012, 09:56   #15 (permalink)
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This has rattled around these hallowed forums for as long as I can remember. There are basically two sorts of pilots.

One who need to 'paint by numbers' and have fixed settings for things. Simulator engineers are generally of this breed. They know EXACTLY how much NI/EPR, rudder trim, pitch etc to apply single-engined and because the sim is just a machine they then fly an IMMACULATE s/e ILS while others flail around. The other sort have a feel for the a/c and in the case of a crosswind will do what bubbers says. Both manage fine.

In both cases, ALL should know that simple aerodyamics require a bigger aileron input at rotate, and if it catches them by surprise they have not been taught properly.

Also as said above, the removal of the cross-controls at lift off is very simple and natural and 9 times out of ten leaves the a/c pointing exactly the right way for the drift.
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Old 7th Feb 2012, 10:10   #16 (permalink)
 
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@Tee Emm, there is still something about large control wheel inputs in the FCTM, main concern seems to be maintaining control at close to Vmcg speeds:

Quote:
Smooth rudder control inputs combined with small control wheel inputs result in a normal takeoff with no overcontrolling. Large control wheel inputs can have an adverse effect on directional control near V1(MCG) due to additional drag of the extended spoilers.
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Old 7th Feb 2012, 17:34   #17 (permalink)
 
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"Also as said above, the removal of the cross-controls at lift off is very simple and natural and 9 times out of ten leaves the a/c pointing exactly the right way for the drift."

Quite. My answer to those who have seen LARGE extra aileron inputs during and after rotation I can only say it ain't helped by having a boot of rudder. There is aileron to counter the wing up effect, but there is even more to counter unnecessary rudder. Once lifting off it ain't necessary any more. I'll invite being shot down, but IMHO the FCTM is not the best explanation of how to do it. I disagreed with their approach to stall recovery technique, and guess what.......I still say we are all thinking pilots and even the Bible allows thinking followers.
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Old 7th Feb 2012, 22:29   #18 (permalink)
 
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I'll invite being shot down, but IMHO the FCTM is not the best explanation of how to do it. I disagreed with their approach to stall recovery technique, and guess what.......I still say we are all thinking pilots and even the Bible allows thinking followers.
RAT5, am totally with you there too!
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Old 7th Feb 2012, 23:34   #19 (permalink)
 
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Quote:
I can only say it ain't helped by having a boot of rudder. There is aileron to counter the wing up effect, but there is even more to counter unnecessary rudder.
I agree 100% with this.
I think people who use heaps of aileron end up inputting extra rudder and the whole takeoff roll becomes more of a struggle. If you only input aileron and rudder as needed you don't get this problem and not much of either is required until rotation when a smooth input of aileron keeps the wings level.
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Old 8th Feb 2012, 01:55   #20 (permalink)
 
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framer....

framer's response is essentially what the Airbus FCTM says....only, his is simpler and easier to understand. It is correct.

We should all be required to fly (to do some crosswind training in) a tail dragger. This will dispel myths....

We should use only enough roll control input to maintain wings level.
We should use appropriate rudder to keep the nose straight down the
runway until after liftoff.

Side loads on the main gear can be damaging. Airbus says more than five degrees could possibly damage the gear.

Airbus has several publications that support what is stated above.


Fly safe,

PantLoad
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