Crab or sideslip landing on big jets?

[Ypilot]

I am curious of what people think.

Many airline pilots seem to be doing the crab landing instead of the sideslip.
Do you guys have any explanation on why it is better to use the crab instead of the sideslip?

I guess, excessive bank and inadequate pitch on landing could damage engines?

image.jpg (233.02 KiB) Viewed 2989 times

[Oxi]

This is just a crosswind landing that is shown but it is a lot easier to crab on final, kick some rudder in and land as pictured.

[hst]

From the Boeing 777 FCTM (Flight Crew Training Manual)

Crosswind Landing Techniques
Three methods of performing crosswind landings are presented. They are the touchdown in a crab, the de-crab technique (with removal of crab in flare), 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. ( Sideslip only (zero crab) landings are not recommended with crosswind components in excess of 28 knots. This recommendation ensures adequate ground
clearance ). 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.

Too your specific question, landing airliners in crosswinds (specifically wide bodies) the preferred method in strong crosswinds is the de-crab method. See link below.

https://www.youtube.com/watch?v=mMvLuUJFHYk

[Meatservo]

It depends on the plane, but I think you'll find there is a little misconception going on when it comes to the "difference" between the two techniques. It's true that with pod-mounted engines there is a danger in landing with too much of a bank angle. It's also true that large planes, with greater inertia, will not take off downwind as suddenly as small ones when the crab angle is "kicked out".

However, the thing that every plane has in common at the moment of touchdown is that the controls are not co-ordinated. There is a "sideslip" happening, even if it is only in the very last split-second before the wheels (hopefully the upwind ones) touch.

If you consider the "de-crab" method described above, you see that using aileron opposite to rudder input during the procedure to keep the wings level is recommended. You can see that even if it is just during the last split-second of the landing flare, the pilot is still using uncoordinated control inputs in order to line up the fuselage and check lateral drift using ailerons.

My opinion is that there aren't "two" techniques, as the "de-crab" technique seems to me to be simply the "sideslip" technique applied at the latest sensible moment and modified to keep upwind bank angle to a minimum. Or, looking at it another way, the "sideslip" technique is simply the "de-crab" technique, applied well before touchdown and requiring opposite aileron to compensate laterally. I think that at the very instant of touchdown however, the two "techniques" boils down to the same thing: using rudder to line up with the fuselage and using ailerons to counter any undesired attitude that happens as a result. It all just comes down to how you choose to think about it: whether you consider the aileron or the rudder to be the control that applies the correction, and the aileron or the rudder to be the control that "compensates" for the unwanted effect of the former. It's really all the same.

On videos where you see airliners touching down pretty much sideways, grinding down the runway in a puff of smoke and then at some point lining up with it just before stopping, I wonder if these pilots aren't placing their faith in the ability of their landing-gears to absorb side-loads and whether this technique is exactly in accordance with the recommended one.


One thing that annoys the heck out of me is when you see people employing and demonstrating the so-called "crab" technique on aircraft other than ones that are designed for it... like Dash 8s and Cessnas!

[Rockie]

My opinion is that there aren't "two" techniques, as the "de-crab" technique seems to me to be simply the "sideslip" technique applied at the latest sensible moment and modified to keep upwind bank angle to a minimum.

True. It's also less uncomfortable for passengers, but de-crabbing has to be done smoothly in a large airplane because you're moving the passengers in the back of the plane sideways a considerable distance in a strong wind.

[hst]

I wonder if these pilots aren't placing their faith in the ability of their landing-gears to absorb side-loads and whether this technique is exactly in accordance with the recommended one.

Touchdown in a crab only condition is not recommended when landing on a dry runway in strong crosswinds.

[Roar]

This is directly out of the Falcon 2000EX EASY CODDE 2.

Landing
During the final approach, fly a wing-level crab into the wind to track the extended runway centreline.
- Maintain the ailerons neutral.
- During flare, just before touch down, cancel the crab angle using the rudder. It is important to set the main gear on the ground quickly after de-crab to minimize drift.
- As soon as possible after touch down, put and keep the nose gear down on the runway.
- Use just the necessary yoke pitch input to prevent the nose to rise avoiding full forward yoke deflection. Use the nose wheel steering to assist the rudder, as necessary.

First point, fly the plane as described in the manual.
Second point, in the case of the Falcon 2000EX EASY, Dassault says to "set the main wheels on the ground quickly after de-crab to MINIMIZE drift." That implies that Dassault has designed and accepted some side load on cross-wind landings.
Lastly, every plane type will have its own procedure..... See first point.

[Meatservo]

It's EASy (sorry) to see why that technique is recommended on the Falcon 2000: No dihedral to speak of and very short gear legs. It would be very EASy (sorry again) to drag a wingtip on that aircraft.

[Roar]

It's EASy (sorry) to see why that technique is recommended on the Falcon 2000: No dihedral to speak of and very short gear legs. It would be very EASy (sorry again) to drag a wingtip on that aircraft.

You are totally correct. On the initial course they told us that the risk of hitting a wingtip was much greater than the risk due to side loading on the gear.

[ahramin]

Depends on the aircraft. Airbus says:

Flare Technique

The objectives of the lateral control of the aircraft during the flare are to land on the centerline, and to minimize the loads on the main landing gear. During the flare, rudder should be applied as required to align the aircraft with the runway heading. Any tendency to roll downwind should be counteracted by an appropriate input on the sidestick (or control column, as applicable). In the case of a very strong crosswind, the aircraft may be landed with a residual drift / crab angle (maximum 5°) to prevent an excessive bank (maximum 5°). Consequently, combination of the partial decrab and wing down techniques may be required.

With higher crosswind (typically above 15 kt to 20 kt crosswind component), a safe crosswind landing requires:
− A crabbed-approach, and
− A partial decrab prior to touchdown, using a combination of bank angle and crab angle (achieved by applying cross-controls).
On most Airbus models, this requires touching down with:
− Maximum 5 degrees of crab angle, and
− Maximum 5 degrees of bank angle.

[KAG]

We decrab the 737 with a touch of sideslip at the last 10/20 feet. Gotta be careful with the split scimitars not to dip the wing too much. On sloppy runways its fine to land the 737 fully crabbed, but I'm not partial to it.

[share-once]

that was my airplane
I usually put a wing down a bit or a slight combination.
Sitting left somehow the sideslip seemed easier with the wind also from the left.

With a decrab you really have to be careful as your upwind wing comes forward.
I took over the controls once on the 737 when the fo didn't keep the wing down. scared the heck out of both of us.
With the 747 and the 777-300 you had to be even more careful as the rear pax are a fair distance behind the gear and they will get a bit of a jolt with de-crabbing (never mind the fa's even further behind )

with the sideslip you have to watch out not to get a pod strike. Mind you you need 7degrees or so to get it and even 2 degrees looks severe from the cockpit.

Thanks for the pic of my ex-bird made my day.
(well...actually the G&T at the pool did, but it's a close 2nd....lol)

[Ypilot]

Wow, thanks for the great answers!
It is interesting to note all the differences with the aircraft models and variable on each such as winglets.

[trampbike]

I wonder if these pilots aren't placing their faith in the ability of their landing-gears to absorb side-loads and whether this technique is exactly in accordance with the recommended one.

Touchdown in a crab only condition is not recommended when landing on a dry runway in strong crosswinds.

As with most things in life: IT DEPENDS

Even the tiny T-38 is to be landed in crab, without any sidesliping, even at the last second.