Sideslip or De-crab in the flare

[typhoonpilot]

There is a good discussion going on in a thread on PPW:

http://forums.propilotworld.com/showthread.php?98159-Technique-for-X-wind-and-HUD

Just thought I would expand the discussion to here and include a poll.


Typhoonpilot

 

[Matt13C]

Where is option D, transition from crab to a slip for the flare?

 

[Screaming_Emu]

Is "doing all of the above poorly" a valid option?

 

[typhoonpilot]

Where is option D, transition from crab to a slip for the flare?

I think what you are referring to is the De-Crab in the flare method, at least in a jet. There is no way you can transition from crab to a full sideslip condition below 50 feet in a jet airliner. At least not without serious risk of striking a wingtip, engine nacelle, or trailing edge of the flaps on the runway.


Typhoonpilot

 

[derg]

330: press the rudder to straighten it out below 50'
320: Sidestick tap, rudder, tap it again, rudder, something (???!!!!!!) *bam!*

 

[typhoonpilot]

For those who don't have access to the link in the first post here are some thoughts. First is Boeing's written techniques from their 737 flight crew training manual:

Crosswind Landing Techniques

Three methods of performing crosswind landings are presented. They are the de-crab technique (with removal of crab in flare), touchdown in a crab, and the sideslip technique. Whenever a crab is maintained during a crosswind approach, offset the flight deck on the upwind side of centerline so that the main gear touches down in the center of the runway.

De-Crab During Flare

The objective of this technique is to maintain wings level throughout the approach, flare, and touchdown. On final approach, a crab angle is established with wings level to maintain the desired track. Just prior to touchdown while flaring the airplane, downwind rudder is applied to eliminate the crab and align the airplane with the runway centerline.

As rudder is applied, the upwind wing sweeps forward developing roll. Hold wings level with simultaneous application of aileron control into the wind. The touchdown is made with cross controls and both gear touching down simultaneously. Throughout the touchdown phase upwind aileron application is utilized to keep the wings level.

Touchdown In Crab

The airplane can land using crab only (zero sideslip) up to the landing crosswind guideline speeds. (See the landing crosswind guidelines table, this chapter).

On dry runways, upon touchdown the airplane tracks toward the upwind edge of the runway while de-crabbing to align with the runway. Immediate upwind aileron is needed to ensure the wings remain level while rudder is needed to track the runway centerline. The greater the amount of crab at touchdown, the larger the lateral deviation from the point of touchdown. For this reason, touchdown in a crab only condition is not recommended when landing on a dry runway in strong crosswinds.

On very slippery runways, landing the airplane using crab only reduces drift toward the downwind side at touchdown, permits rapid operation of spoilers and autobrakes because all main gears touchdown simultaneously, and may reduce pilot workload since the airplane does not have to be de-crabbed before touchdown. However, proper rudder and upwind aileron must be applied after touchdown to ensure directional control is maintained.

Sideslip (Wing Low)

The sideslip crosswind technique aligns the airplane with the extended runway centerline so that main gear touchdown occurs on the runway centerline.

The initial phase of the approach to landing is flown using the crab method to correct for drift. Prior to the flare the airplane centerline is aligned on or parallel to the runway centerline. Downwind rudder is used to align the longitudinal axis to the desired track as aileron is used to lower the wing into the wind to prevent drift. A steady sideslip is established with opposite rudder and low wing into the wind to hold the desired course.

Touchdown is accomplished with the upwind wheels touching just before the downwind wheels. Overcontrolling the roll axis must be avoided because overbanking could cause the engine nacelle or outboard wing flap to contact the runway. (See Ground Clearance Angles - Normal Landing charts, this chapter.)

Properly coordinated, this maneuver results in nearly fixed rudder and aileron control positions during the final phase of the approach, touchdown, and beginning of the landing roll. However, since turbulence is often associated with crosswinds, it is often difficult to maintain the cross control coordination through the final phase of the approach to touchdown.

If the crew elects to fly the sideslip to touchdown, it may be necessary to add a crab during strong crosswinds. (See the landing crosswind guidelines table, this chapter). Main gear touchdown is made with the upwind wing low and crab angle applied. As the upwind gear touches first, a slight increase in downwind rudder is applied to align the airplane with the runway centerline. At touchdown, increased application of upwind aileron should be applied to maintain wings level.


TP

 

[typhoonpilot]

Here is a video of De-Crab in the flare on a widebody airliner:

TP

 

[Screaming_Emu]

330: press the rudder to straighten it out below 50'
320: Sidestick tap, rudder, tap it again, rudder, something (???!!!!!!) *bam!*

 

[typhoonpilot]

It will be interesting to see if this thread results in some mature intelligent discussion on the topic or devolves into juvenile jokes and silly posts .


TP

 

[TwoTwoLeft]

It depends....

Sideslipping on final creates more drag. Carrying more power is required to avoid excessive sink rates. Not really something you want if LLWS might be an issue. Especially in a large/jet aircraft. A common justification for the technique is to "see how much crosswind correction you will need before you land". For that to be valid, the crosswind must be the same on final where the inputs are made all the way down to the touchdown point. As we all know, that's seldom the case. Throw in mechanical turbulence and why even bother? I personally only use this as a training excercise for the pilot to develop a muscle memory of the control inputs while tracking the runway centerline to the ground.

Crab or "kickout" in the flare is better energy management. Pilots with proper hand-foot-eye coordination and experience in their aircraft type should be able to make crosswind landings a non-event.

Landing in a crab... There may be a few exceptions for contaminated runway surfaces. Generally, unless your airplane is an Ercoupe, equipped with crosswind landing gear, trailing links for the mains or structurally approved, landing crabbed is a really bad idea in anything from a J3 cub to a jet. Its hard on everything from tires to structure and it just looks like amateur hour. You'll eventually need to apply proper control inputs on the roll out, so why not just apply them correctly in the flare?

 

[Screaming_Emu]

It will be interesting to see if this thread results in some mature intelligent discussion on the topic or devolves into juvenile jokes and silly posts .


TP

I see no reason as to why we can't have both.

 

[deadstick]

I'm but a lowly King Air pilot and use the de-crab, but sometimes it doesn't work, and I end up with a bit of a side slip at the very end. Then there are some who try to de-crab and still land in a crab because their sight picture is messed-up. They are also the ones who land diagonally down the runway because their final is 5+ degrees or so off the centerline.

 

[TwoTwoLeft]

Depending on amount of crosswind and given control authority of the airplane, landing wings level may not be enough to cancel out the lateral drift.

There really is nothing wrong with touching down in the upwind main first, provided that is what it is going to take to touch down with zero drift and your longitudinal axis align with your direction of travel. The only limitations would be wing tip or engine clearance issues. Usually those aircraft have trailing link gear, so touching down with a slight crab is preferred rather than trying to prevent lateral drift.

In a twin you can use asymmetrical trust to your advantage. Carrying a little more power on downwind engine works great in a stiff crosswind. Your PAX might not like the sound of it, but they appreciate a smoother landing.

 

[Polar742]

In the 747 (all variants), most of the time I'd kick out the crab and set a sideslip to land. On but a handful of landings where the wind was very strong this method worked fine.

The 767 seems to be the same so far.

YMMV

 

[seagull]

I think what you are referring to is the De-Crab in the flare method, at least in a jet. There is no way you can transition from crab to a full sideslip condition below 50 feet in a jet airliner. At least not without serious risk of striking a wingtip, engine nacelle, or trailing edge of the flaps on the runway.


Typhoonpilot

Trying to understand why this would be true? Just adding rudder until aligned with the runway and lowering the wing to counter the drift is a "full slip" is it not? It should not result in wing structure or a pod contacting the ground unless the wind is way above demonstrated values unless I am not understanding what you said?

 

[seagull]

It depends....

Sideslipping on final creates more drag. Carrying more power is required to avoid excessive sink rates. Not really something you want if LLWS might be an issue. Especially in a large/jet aircraft. A common justification for the technique is to "see how much crosswind correction you will need before you land". For that to be valid, the crosswind must be the same on final where the inputs are made all the way down to the touchdown point. As we all know, that's seldom the case. Throw in mechanical turbulence and why even bother? I personally only use this as a training excercise for the pilot to develop a muscle memory of the control inputs while tracking the runway centerline to the ground.

Crab or "kickout" in the flare is better energy management. Pilots with proper hand-foot-eye coordination and experience in their aircraft type should be able to make crosswind landings a non-event.

Landing in a crab... There may be a few exceptions for contaminated runway surfaces. Generally, unless your airplane is an Ercoupe, equipped with crosswind landing gear, trailing links for the mains or structurally approved, landing crabbed is a really bad idea in anything from a J3 cub to a jet. Its hard on everything from tires to structure and it just looks like amateur hour. You'll eventually need to apply proper control inputs on the roll out, so why not just apply them correctly in the flare?

Not much of a factor. The widebody Douglas airplanes recommended technique is to transition into a side slip at about 300 feet. The reason for doing it that high up is just to allow the autothrottle time to catch up with the drag increase. In truth, if you bump up the power as you cross it up it doesn't matter much.

 

[matthew]

330: press the rudder to straighten it out below 50'
320: Sidestick tap, rudder, tap it again, rudder, something (???!!!!!!) *bam!*

So options D: Slam it on

 

[typhoonpilot]

Trying to understand why this would be true? Just adding rudder until aligned with the runway and lowering the wing to counter the drift is a "full slip" is it not? It should not result in wing structure or a pod contacting the ground unless the wind is way above demonstrated values unless I am not understanding what you said?

The answer is, no it is not. The de-crab in the flare technique, which is the one advocated by both Airbus and Boeing on all their widebody aircraft is done after the flare has commenced, just prior to touchdown.

The bank angle would not be the same as a de-crab in the flare as it is in a sideslip. With de-crab in the flare you only add a little bit of aileron to keep the wings level while still airborne. Done properly, just prior to touchdown, there is not as large a requirement for aileron because the aircraft touches down before the full affect of the rudder becomes an issue. The upwind wing will have started sweeping forward but the actual roll that causes just doesn't have time to develop. After touchdown the aileron input can be increased but the wheels are already on the ground so the danger of striking the wing tip, engine nacelle, or flap are no longer there.


Typhoonpilot

 

[seagull]

The answer is, no it is not. The de-crab in the flare technique, which is the one advocated by both Airbus and Boeing on all their widebody aircraft is done after the flare has commenced, just prior to touchdown.

The bank angle would not be the same as a de-crab in the flare as it is in a sideslip. With de-crab in the flare you only add a little bit of aileron to keep the wings level while still airborne. Done properly, just prior to touchdown, there is not as large a requirement for aileron because the aircraft touches down before the full affect of the rudder becomes an issue. The upwind wing will have started sweeping forward but the actual roll that causes just doesn't have time to develop. After touchdown the aileron input can be increased but the wheels are already on the ground so the danger of striking the wing tip, engine nacelle, or flap are no longer there.


Typhoonpilot

Understand that, but... In any Boeing, Airbus or even the low slung DC8 70 series the max bank for a side slip would not cause a strike even well over the max demo wind.

 

[typhoonpilot]

Understand that, but... In any Boeing, Airbus or even the low slung DC8 70 series the max bank for a side slip would not cause a strike even well over the max demo wind.

I'd have to disagree. In the Boeing Flight Crew Training manual on all models they specifically mention a crosswind component above which one should not use full sideslip technique. That is there specifically to prevent contacting a portion of the aircraft with the ground. For example in the B787 the maximum demonstrated crosswind on landing is 33 knots. The FCTM cautions against using sideslip only technique above a 25 knot crosswind component: "This recommendation ensures adequate ground clearance.....".

There have been a number of tip strikes on Global 6000s and other business jets recently so this is a rather important topic.

Different airlines in the USA have different cultures in regards to these techniques. NWA was adamantly a de-crab in the flare airline, while USAir (at least on the narrowbody fleets) was more of a sideslip airline.

The sideslip technique is peculiarly a North American and South American technique. You will not find pilots in Europe, Australia, New Zealand, South Africa, or many other countries using anything other than de-crab in the flare.


Typhoonpilot