Photo: Crosswind landing sequence, from Bernal Saborio (Creative Commons).
As pilots we learn to deal with crosswind landings from day one. Chances are this was in a primary flight trainer that weighed less than your car and was accomplished with what some call the "wing low" method, something more properly called a sideslip. This could very well be a valid technique for the jet you are flying now, but it may not be. There are three basic techniques to choose from:
Regardless of method, a common goal is to have the airplane aligned with the runway following touchdown. Each method has its advantages and disadvantages. If your aircraft is allowed more than one method, you should know the pros and cons of each. If your aircraft manufacturer specifies a particular method, you should use that method and understand its limitations. I've flown aircraft that require landing in a crab (T-38), mandate landing only with the sideslip (B-707, T-37), allow you to use any method with limitations placed on the sideslip (B-747), or require a decrab just prior to touchdown (CL-604, GIII, GIV, GV, G450).
What follows comes from the references listed below, with my comments and techniques shown in blue. I don't think anyone explains the techniques better than Boeing, and most late model Boeings allow all three methods with a few limitations. I've used the Boeing 737 NG Flight Crew Training Manual to introduce each technique as a starting point, with other aircraft offered for another point of view. If your manuals are silent or sparse when discussing cross wind landings, perhaps what follows can fill in the gaps.
I've also included some other comments: Other Perspectives.
Figure: B-737 touchdown in a sideslip, from Eddie's notes.
The following 737 technique can be found in other Boeing manuals and may not apply, word-for-word, to other aircraft that allow touchdown in a sideslip. But if your manual doesn't offer any advice at all, or if you are looking for how "the big boys" do it, it gives you more information than you had.
[Boeing 737 NG (TM), page 6.46]
[Boeing 737 NG (TM), page 6.45] Sideslip only (zero crab) landings are not recommended with crosswind components in excess of 26 knots. This recommendation ensures adequate ground clearance and is based on maintaining adequate control margin.
The sideslip technique tends to be restricted to aircraft with short, unswept wings . . .
[Pilatus POH, page 10-1] It is recommended to use the wing down technique. At approximately 100 to 200 ft on approach to the runway, apply rudder to align the longitudinal axis of the aircraft to the runway and put on bank in the opposite direction to maintain the runway centerline. The aircraft is the flown in a sideslip to touch down initially on one wheel. As soon as one wheel touches, lower the other two to the runway and immediately select either the condition lever to ground or the PCL to beta or reverse.
The Air Force Boeing 747-200 that I flew (E-4B) allowed touchdown in a sideslip and no crab angle up to 25 knots of crosswind. As crab angle is increased, the crosswind limit went up to 40 knots and higher. Why the limit? We were told we could actually hit the runway with an engine pod. Because of the swept wing, the chances of dragging an engine nacelle went up as the speed went down. Many business jets have highly swept wings that put the wing tips at risk, especially at high deck angles . . .
Figure: GV wing tip clearance in a 3-point attitude versus 5° nose up, from Eddie's notes.
Looking at your aircraft in a three point attitude may lead you to believe it will take a lot of bank to get the wing tip in a sideslip landing. If you have a swept wing, the look of things can be deceiving. In a GV, for example, the wing tip is actually lowered 17" when the nose is raised to a normal flare attitude. More about this: Technical / Flight Math / Wing Tip Hit Bank Angle on Landing.
Figure: GV bank angle to hit a wing tip in a 3-point attitude, from Eddie's notes.
In a 3-point attitude, if you were able to push the aircraft over on a wheel, the wing would contact the ground at 7° of bank:
Figure: GV bank angle to hit a wing tip in the landing attitude, from Eddie's notes.
In a normal landing attitude, the wing tip is lower to the ground and you would hit with only 5° of bank:
Note that with any yaw into the lower wing, the wing tip strike would happen even sooner.
Figure: B-737 touchdown in a crab, from Eddie's notes.
The following 737 technique can be found in other Boeing manuals and may not apply, word-for-word, to other aircraft that allow touchdown in a crab. But if your manual doesn't offer any advice at all, or if you are looking for how "the big boys" do it, it gives you more information than you had.
[Boeing 737 NG (TM), page 6.45]
Longer aircraft, such as the Boeing 747, 757, and 767, must also consider the distance of the cockpit versus the main gear:
[Boeing 767 (TM), page 6.37]
[Cessna Citation X Model 750 Airplane Flight Manual, page 3-205.]
While the Citation X calls their recommended crosswind landing technique a crab, it is actually a de-crab in the flare up to a point after which a crab is suggested at higher crosswinds.
Figure: Computing crosswind crab angle, from Eddie's notes.
If your aircraft allows you to land in a crab, the maximum (demonstrated or limiting) crosswind will determine just how much of side load the manufacturer anticipated from these types of landings.
If your aircraft requires a sideslip or a de-crab prior to touchdown, knowing how much crab is allowed prior to removal will give you an excellent gauge of how much crosswind you really have.
The angle is found by dividing the crosswind component by the approach speed, and then pulling out a scientific calculator and looking for the inverse sine function, seen usually as "arcsin" but sometimes "sin-1." I've done a few for you here:
|X-W||VAPP = 105||VAPP = 130|
Figure: B-747-200 cockpit displacement example, from Eddie's notes.
Long aircraft have an additional challenge when landing in a crab: cockpit displacement. The aircraft will pivot around the main landing gear on touchdown so pilots must place their eyes upwind of the runway centerline. A Boeing 747-200, for example, has two sets of main landing gear. The furthest aft set, known as the body gear, will touch down first and the aircraft nose will tend to swing forward. Using our crab angle = sin-1 (XW / VAPP) formula, we see that the amount of displacement can be significant.
Figure: Reverse thrust and crosswind, from Eddie's notes.
[Boeing 767 FCT, page 6.35]
In the world of motorcycles we say the tires only have so much grip and if you use up grip to brake you have less to corner. With airplanes this also applies, you can do both at the same time but not as well as you can do one or the other.
Note: Use of this technique increases the required landing distance.
Figure: Decrabbing a B-737, from Eddie's notes.
The following 737 technique can be found in other Boeing manuals and may not apply, word-for-word, to other aircraft that specify a de-crab of some sort. But if your manual doesn't offer any advice at all, or if you are looking for how "the big boys" do it, it gives you more information than you had.
[Boeing 737 NG (TM), page 6.45]
[Bombardier BD-700 Flight Crew Operating Manual, §04-08-15, ¶10.]
[Challenger 604 Operating Manual, pg. 06-13-5] In crosswind conditions, the crosswind crab angle should be maintained for as long as possible until prior to touchdown. Aim for the centerline or slightly on the upwind side and avoid touching down on the downwind side of the runway because of the possibility of the airplane weathervaning towards the wind after touchdown, and drifting toward the downwind side of the runway.
[Falcon 2000EX EASy CODDE, §03-15-40, page 1] Landing
[G450 Aircraft Operating Manual, §06-05-10, ¶2.B.]
Gulfstream eliminated the entire section providing landing procedures and techniques a few years ago, what follows is from the September 2009 version of the operating manual. I am told they are leaving these discussions to the training centers. It is my experience that the training centers don't address the issue at all. So here is what the book used to say.
[The following comes from a major airline's Airbus 300 pilot's manual]
Photo: Aircraft wreckage, from NTSB AAR 0501, figure 2.
The first challenge in a de-crab is getting the timing just right. If you de-crab too early you will need more bank angle and risk a wing tip or engine pod strike. If you de-crab too late, you risk landing in a crab. The second challenge is to shift your mind from flying in a crab to flying aligned with the runway. Even if your flight manual calls for an early de-crab, you need to continue to fly the airplane aligned with the ruwnay.
In 2003 a FedEx MD-10 crash landed in Memphis when the first officer de-crabbed at the appropriate time and then removed all sideslip controls prior to touchdown. I believe the subsequent aircraft drift caused her to freeze at the controls and forget to flare. The very hard impact was made with the aircraft in a 6° crab, causing the right landing gear to collapse. The aircraft was destroyed, there were two injuries, no fatalities. More about this: Mishaps / Federal Express 647.
During my five years flying the Boeing 707, landing in anything but a sideslide was considered heresey. Of course that made for some sporting crosswind landings but my technique was to fly the airplane onto the runway and eliminate hovering over the runway while letting the wind drift me or risking an engine pod or flap. Barry Schiff's method is an optimal de-crab and looks brilliant. If, like me, you prefer to fly the airplane onto the runway rather than flare to zero inches, the method works very well so long as you err on the side to early rather than late.
The kickout method
November 1, 2001
By Barry Schiff
Barry Schiff has been flying for 49 years. He retired as a captain from TWA in 1998.
The first jet airplane I flew was a TWA Boeing 707-100, an ancient model that was pure turbojet (not turbofan) with water injection to boost takeoff performance. We called it the Water Wagon.
Just looking at the airplane made me wonder how it was possible to make crosswind landings without dragging a pylon-mounted engine pod on the runway. (In reality, the first thing that would scrape the ground would be the aft, inboard corner of a wing flap.) Pilots probably have wondered the same about the Boeing 737. Its pods are so close to the ground that the bottoms of its nacelles are flattened on later models to increase ground clearance.
My curiosity was satisfied when I sat in a cockpit jump seat on a 707 flight to Kansas City, Missouri, in 1964. The captain had established a considerable crab angle while on final approach to compensate for the 29-knot left crosswind component, the maximum allowable for that airplane. We crossed the runway threshold while still crabbing, and I began to wonder when the captain would transition the airplane into a wing-low slip, the time-honored method of making a crosswind landing. But he held the Water Wagon in a crab even as he pulled the throttles back and began to flare. "Omigod," I thought. "We're going to land sideways and shear the tires from their wheels." I looked at the copilot, and he didn't appear concerned.
Just as the tires on both sides of the airplane were about to touch, the captain applied right rudder to straighten the airplane and simultaneously cranked the control wheel hard to the left. The airplane banked into the crosswind, and the left main landing-gear wheels kissed the concrete.
I had just witnessed a masterfully executed crosswind landing utilizing what I was to learn is called the kickout method.
Although it might seem at first blush that this method involves simply delaying the transition from crab to slip until the last second, there is much more to it than that. Instead of waiting to slip and then landing, this technique involves a transition to a slip that is interrupted by a landing. In other words, the airplane is so close to the ground while still crabbing that it is the act of lowering the wing that forces the upwind tires to touch down firmly and positively. It is the act of banking that causes touchdown. The goal is to plant the upwind gear on the ground and thus eliminate the need to enter the steeply banked slip that would otherwise be required to prevent drift.
In this manner, the pilot doesn't have to sit and wait for the aircraft to plop down while hovering in a flare above the runway in a steep, uncomfortable slip. By lowering the upwind wing while the aircraft is only inches above the runway, the upwind tire is forced down to make firm contact with the ground.
It is a simple matter to determine when the aircraft has been banked sufficiently; just listen or feel for ground contact. Simultaneous with the application of upwind aileron, it is necessary to kick the aircraft out of the crab with firm and opposite downwind rudder so that the wheels are aligned with the runway at the instant of touchdown.
Once the upwind tire is forced onto the ground, aileron pressure should be increased to keep it there. Rudder pressure continues to be applied as necessary to maintain runway heading.
The final step is the same as always. Allow the downwind tire to touch, followed by the nosewheel or tailwheel.
After I had learned to make this kind of a crosswind landing in the big jets, I had wondered if the same technique could be used in light airplanes. To my delight, I discovered that this somewhat unorthodox method of landing crosswind is equally useful in general aviation aircraft, even if they don't have engine pods slung under their wings.
I was heading home in a Piper Cherokee 140 on a day when the wind was gusting across Runway 3 at 30-plus kt. An attempt at a conventional crosswind landing failed because I could not slip steeply enough on short final to offset drift. I was about to divert to a nearby airport when I decided to try the kickout method for the first time in a general aviation airplane.
I held a tremendous crab angle on final and was grateful that there was no such thing as a maximum-allowable crab angle. There is, of course, a definite limit to the degree of slip that can be maintained in any given airplane.
I held the crab until only inches above the runway and checked the descent with a touch of back-pressure. When the tires were only seconds from scrubbing, I simultaneously kicked out of the crab and lowered the left wing. The left tire plunked onto the runway. The simple act of banking forced the landing. Because of the brutality of that crosswind, I pushed the aircraft over onto all three legs and braked to a stop.
The kickout method isn't for everyone. But for those who want to try something new and effective, wait for a day when the windsock points across the runway, hire a proficient instructor to ride shotgun, and give it a try. You'll probably have the airport to yourself (and the tower controllers thinking that there is a lunatic loose in the traffic pattern).
Congratulations on your excellent article on cross-wind landings published in the June 2015 issue of B&CA. I plan on giving it to my students during their type rating training on the Boeing 737 Classic simulator where I am a simulator instructor on that type. I retired from active 737 flying many years ago but fortunately still instruct in the simulator and conduct instrument ratings.
I learned to fly in the Royal Australian Air Force in 1951 and went on to fly the P51 Mustang and single seat De Havilland Vampires plus the Hawker Sea Fury. During my 18 year military career I also instructed on the Avro Lincoln four engine heavy bomber (an advanced and heavier edition of the wartime Lancaster) and also on the DC3, Convair 440, Vickers Viscount and other military and civil types. I hold an Australian ATP as well as instrument rating and fly the Texan 600 LSA as a hobby. I also write occasional articles for the www.airfactsjournal.com website.
The main reason for writing to you is to offer some thoughts on crosswind landing techniques on the Boeing 737 Classics. I have about 8000 hours on the 737-200 to 500 series.
I have always thought that not enough attention is paid by instructors in the simulator on crosswind landing techniques. While accent on automation seems to be the priority during type rating training (rightly or wrongly in my view), this should not be at the expense of raw data manual flying skills. On many occasions I have seen pilots who are highly proficient at using the automatics, but seem hopeless at flying without the aids of the autopilot, autothrottle and flight director. This is because their companies discourage anything except full use of automation. I believe this is a world- wide problem particularly in Asia, Africa and the Middle East where many first officers are cadet pilots with very little manual flying experience.
This really shows out during crosswind handling which is probably why some airlines place a crosswind component limit for first officers. Instead of spending more simulator time on training for crosswind landings up to the aircraft limit, especially during initial type rating training, the tendency for instructors is to quickly tick the box after one or two crosswind landings and move on to something else. This causes lack of confidence in new pilots to type and captains therefore are often reluctant to allow first officers to land in even moderate crosswinds of 15-20 knots. The problem is exacerbated if the flight recorder is used as punitive device which is common in Asia. Air China, for example, applies fines to the pilot if the QAR shows a pilot is caught taxiing two knots over the limit for taking corners. Another Chinese operator, Hainan Airlines, mandates that first officers on the A330 are not permitted to take off or land until five years on type have elapsed. This was considered necessary after a series of rotation and landing flare tail strikes by very inexperienced 250 -500 hour first officers.
In the simulator when introducing normal dry runway crosswind landings, I find it common to witness pilots slow to correct for drift on final and finally edging in towards the extended runway centreline rather than make early corrections to track the centreline. Initially pilots often touch down with drift still on because they failed to allow for inertia as they attempt to remove drift. Some pilots cannot “see” drift and deny having drift on touch down. I find the fix for that is to freeze the simulator at the second the wheels hit the runway. I then point out to the student the obvious difference between aircraft heading as against runway direction. Eventually the student realises he has a problem and practice is needed. The quickest method is to re-position the simulator at one mile final and practicing the last part of the approach and touch down until the student gets the hang of it and can touch down with no drift. Usually several attempts are needed until the student is competent. Later, crosswind approaches on instruments should be practiced from a five mile final. It is quite possible that up to one hour of simulator time on crosswind landings are needed by the time frequent re-positioning takes place.
You made the point that landing with crab on slippery runways can pose problems with reverse thrust directional control. You also go on to say that pilots may have no choice but to return the reverse thrust to idle, return to the centreline, and re-attempt reverse thrust once the aircraft is aligned with the runway. The Boeing 737 FCTM adds that to correct back to the centreline the first thing is to release the brakes and then reduce reverse thrust to reverse idle.
As you well know, all this requires good handling skill and knowledge of the forces at work. This is where proper simulator instruction is needed and it has to be accepted by training managements it will be time consuming until the candidate can consistently display competent handling of slippery runway crosswind landings. In fact it is my experience that at least ten slippery runway crosswind landings are needed in the simulator before candidates become confident of a safe landing. It is best conducted raw data (no FD) and manual throttles and no autopilot.
Looking first at the Boeing FCTM technique of first releasing the brakes. The autobrakes will probably be selected on unless for some reason manual braking is to be used. In the 737 the autobrakes can be de-selected by three means at pilot discretion. 1. Lowering the speed brake lever. 2. Depressing the rudder pedal brakes until the autobrake disengages after a set pedal pressure. 3 Switching off the autobrake selector switch. It seems completely foreign in nature to deliberately release the brakes when they are most needed which is on a slippery runway surface. But that is the first action to prevent the crosswind slide. To place the speed brake lever down to release the brakes is counter-productive in that drag is immediately reduced and weight on the wheels is reduced.
Over-riding the autobrakes into manual braking by depressing the rudder pedals may be counter- productive, since that means applying harder brakes which has the potential for worsening the side- ways slide, albeit momentarily. That leaves actuation of the autobrake selector to off, which has none of the disadvantages of the previous two choices. For several seconds until the aircraft is aligned with the runway there is no wheel braking. The next step is to select reverse idle to prevent the tail from weathercocking because of reverse thrust vectors. These actions (brakes off and reverse idle selected) are done simultaneously.
Assuming full reverse had already been applied when the tail starts to slide sideways, it takes at least ten seconds from 90%N1 (typical full reverse) to reach 23% N1 which is reverse idle in the CFM 56 engine. This is typical spool down time. If the reverse idle detent on the throttle quadrant is not immediately apparent in the heat of the moment, then it can happen the pilot will inadvertently go too far down with the reverse levers and go into forward thrust; an obviously undesirable situation especially as the brakes are already released.
Then comes the next step which is to use rudder to steer towards the centreline. Nosewheel steering at high speed is inadvisable and may be prohibited in some aircraft, especially on a slippery runway. All this takes time. Once angling towards the centre line the pilot has to decide whether to re-apply full reverse again to obtain some retardation or should he have the aircraft straight down the centreline before applying full manual braking along with re-application of full reverse thrust.
In the CFM 56 engine to obtain full reverse from ground idle reverse of 23% N1, takes at least seven seconds. Meanwhile speed brakes and hard manual braking (with anti-skid working overtime on a slippery runway) are the only effective retarding force until full reverse winds up to maximum. As full reverse is ineffective below 80 knots, the long delay that occurs from first reaching idle reverse as the aircraft started to slide sideways, then reapplying full reverse once the aircraft has straightened up, means the primary retarding force is manual braking. I doubt if the autobrakes can be re-engaged once they are disabled.
The amount of runway used in all this will clearly be much longer than for a slippery no crosswind landing. Thus performance calculations for a slippery runway are invalid and the pilot is largely in no-man’s land re stopping distance. These factors need to be considered if faced with a crosswind landing on a slippery runway. It would be wiser to pick an into wind runway or divert.
All the above requires a thorough briefing before training starts in the simulator. This includes among other things, practicing (static in the simulator with brakes on) the technique of quickly applying reverse on touch down and then quickly reverting to idle reverse from full reverse. Timing of spool down and spool up times should be made so that pilots are aware of the long delays reaching idle reverse and full reverse from idle. Some pilots are used to slowly selecting reverse idle on a normal landing. In the case of a slippery crosswind landing the pilot must lower the reverse thrust levers very quickly to the idle detent to reach idle reverse promptly to prevent weather cocking due to high reverse.
But before the slippery runway with crosswind is introduced in simulator training, the pilot under training must be competent at normal max crosswind landings. That includes first officers who must be trained to be equally competent in aircraft handling in these conditions, as the captain. It brings up the question of priority during type rating or recurrent training; that is automatics training first Or learn to fly the aircraft manually until competent in all handling sequences with automatic pilot training being introduced later. I prefer the latter and have always trained my candidates that way.
Boeing 737 NG Flight Crew Training Manual, Revision 12, June 30, 2013
Boeing 767 Flight Crew Training (FCT) Manual, June 30, 2013
Bombardier Global Express BD-700-1A-10 Flight Crew Operating Manual (FCOM), CSP 700-6, Volume 1, Revision 78, Nov 19, 2013
Cessna Citation X Model 750 Airplane Flight Manual, Revision 2, 10 April 2012
Challenger 604 Operating Manual, Publication No. CH 604 OM, Rev 56, Jan 14/05.
Falcon 2000 EX EASy Crew Operational Documentation for Dassault EASy I (CODDE), Issue 9, November 30, 2013
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
NTSB Aircraft Accident Report, AAR-05/01, Federal Express Flight 647, Boeing MD-10-10F, N364FE, Memphis, Tennessee, December 18, 2003
Pilatus Pilot's Operating Handbook, 27 July 2010.