Assymetrical Thrust proper tech in x-wind?
 

Question for the masses: Is the use of assymetrical thrust a valid technique for control about the vertical axis in a crosswind in any jet, large-light? Go...

For the record, I am of the school that it is not but flew with someone who swore by it and looked at me as if I were crazy for not subscribing to it. Not that I am an expert, but this is something I've never seen taught and I'm sure there are aerodynamic consequences that make it unadvisable. However, I havn't yet put much thought into it so I thought I'd lob it out there.

not for takeoff.

performance is based on all engines at takeoff (or flex) thrust until V1, then a loss of one engine at V1 and takeoff continued, for instance.

if you reduce thrust on one engine, and have not calculated the increase in acceleration distance required to reach V1,

"oh-oh!"

I've used asymetrical thrust in a light twin for landing, but never in a turbine airplane.

Anybody who would use it for takeoff is the definition of careless and reckless, IMO...

Assuming you mean on landing, the answer is no. Some aircraft like DC-8 and B747 have roll limits to protect outboard engines which necessitate landing in a crab. My preferred technique is to hold the crab down to the last foot or so, then kick it out just as the wheels touch--paying particular attention to getting the aircraft on the ground and tracking straight down the runway before it starts drifting. Others actually land in the crab, but as spoilers and reverse don't always operate symetrically you could find yourself fighting the aircraft all the way down the runway, especially if it's slick. Plus, that's awfully hard on the landing gear. You won't need asymetrical thrust to help with any crosswind up to the demonstrated limit, and you'd also need to think about a go-around with half your power alreadly spooled up and the other half a little behind. Now that could get exciting!

As for using it on takeoff, never. That's what rudders and ailerons are for.

I'd be willing to bet money he's talking about landings....

and while not a great deal of time is spent teaching the principle in most flight school license mills.... if you had a real instructor (read that as an instructor who was doing more than teaching to build time to take a shiney jet job), they would have introduced you to the principle during your multiengine training. The problem with trying to teach it in light twins is that it really is a skill that isn't required in that type of airplane... actually, short of meeting or exceeding demonstrated crosswind limitations most aircraft don't need this method.
Fast forward to larger jets with poor engine ground clearance issues and the wing low method can be fairly expensive and hard on engines and wing tips... (have you ever seen the old Hong Kong 747 videos?) you will often see a crab carried into the flare followed by a "kick over" for allignment... in this case you may find some old school guys also adding differential power. It can be an effective tool if used correctly, much as the souix city crash/incident was as successfull as it was based primarilly upon the use of differential power.

nevermind, ought to read what i write before i post it...

How do ya figure that one?

I have never used asymmetrical thrust in a turbine and have never been in a situation where I even considered it, but I have talked to a bunch of B1900 pilots who claimed that during extreme crosswind landings it was definitely a good technique that helped get the airplane down on the ground and through the rollout safely.

Maybe any B1900 pilots can contribute?

And you probably will never see it taught.....

I've got over 3000 hours in the B1900 and have only had to use it a few times. It is a very extreme case that calls for it. It does work though.

Over a 1,000 hours in the 1900 and flew it in the Midwest in winter. Never did I need differental thrust to make a safe landing. Not saying that it isn't possible or that it doesn't work but I never saw anyone use it either.

Great idea in a DC-3 landing with a strong crosswind on a contaminated runway, (icy runway with a 25-30 knot crosswind). Bad idea in a jet.

Hope that helps.

I have landed the Mighty Beech in max X-wind more times than I can count....never needed anything other than full power or idle. The barber pole was your friend. Seriously though, I subscribe to the levers together crowd.

If you're flying a multi-engine jet, no. Put the upwind wing down and use opposite rudder. Do it about 1000 to 500 feet from touchdown. Hold the controls throughout the landing and roll out. Works every time and ensures a nice touchdown.

Never have used it in a jet but I used to use it in the Shorts 360 all the time. It was just a matter of "idleing" the downwind engine in the flare. Seemed to help in an airplane that was crosswind challenged.

It works reasonably well in a sececa. I dont think I would want to try it in a jet.

I disagree. I've used asymmetrical thrust in many in bizjets and the DC-10. Why do you two say it is a bad idea in a jet?

What I don't understand is, once you reduce the thrust on the downwind engine, you will introduce a drift into that direction. To compensate for that drift, you must bank into the upwind engine and/or the wind. (I could be missing something)

I don't understand how reducing power on the downwind engine solves or helps your situation.

As buzzpat said before, forward slip (bank into the wind, downwind rudder) works just fine, even in the big jets.

It definately helps in the Metro.

It's like having a little more rudder. Still need to correct for the drift...

I don't like the forward slip, btw. Increases pax discomfort and decreases performance.

You slip in the flare. If you are smooth, the passengers don't really feel anything.

Pulling power on a good engine certainly is a decrease in performance. And having an asymetrical spool up in case of a rejected landing isn't a good situation.

And as you have said, you don't stop the drift. That's one of the big dangers in crosswind landings is not killing he drift. Landing gear can only take so much side loading before they fail.

What's scary are pilots on here admitting they do this "technique" when it goes against the manufacturer's recommendations on crosswind landings. I definitely would have handed out an "epic fail" to someone had they done this while I was an aircraft examiner.

Boeing describes three crosswind landing techniques;

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

The upwind wing down technique will provide the least amount of torsion type stress to the landing gear as a whole.

While it may not be a specific manufacturers limitation, reducing thrust on the downwind side of the jet may have limiting performance due to go around performance and potential VMC situations from a balked landing at extremely low speeds when flaps are extended beyond the engine inoperative landing position.

Boeing has this to say about touchdown in a crab.

“…the greater amount of crab at touchdown, the larger the lateral deviation from the point of touchdown. For this reason, touchdown in a crab on condition is not recommended when landing on a dry runway in strong crosswinds.”

This particular technique it is not a limitation, nor prohibited as contained in the Boeing Flight Crew Training Manual for the Boeing 727 and 737.

Perhaps other crewmembers can provide information about other Boeings, and other manufacturer specific rather than airline specific such as Airbus, or Embraer.

With that being said, chapter section page contained within your airline's specific flight crew training manual would have to be provided to "Epic Failed" applicant/pilot.

LANDING MODE For the A320 Volumen 3 3.04.27. Pag 4

The system's landing mode gives the aircraft a stabilized flight path and makes a conventional flare and touchdown. It carries out the initial approach as this manual described earlier. At 50 feet, the system memorizes the attitude, usually 3° or 4° nose up. From 30 feet down, this value washes out over eight seconds to - 2°. The result is that the pilot has to exert a progressive pull to increase pitch gently in the flare. He should pull the thrust levers back at or above 20 feet, and the landing should occur without a long flare. Touchdown quality is better and more repeatable at fairly flat attitudes. An audible "RETARD" callout reminds the pilot if he has not pulled back the thrust levers when the aircraft has reached 20 feet.

Crosswind landings are conventional. The preferred technique is to use the rudder to align the aircraft with the runway heading, during the flare, while using lateral control to maintain the aircraft on the runway centerline (Refer to SOP 3.03.22). The lateral control mode does not change until the wheels are on the ground, so there is no discontinuity in the control laws. The aircraft tends to roll gently in the conventional sense as drift decreases, and the pilot may have to use some normal cross control to maintain roll attitude.

Even during an approach in considerable turbulence, the control system resists the disturbances quite well without pilot inputs. In fact, the pilot should try to limit his control inputs to those necessary to correct the flight path trajectory and leave the task of countering air disturbances to the flight control system.

Derotation is conventional. The pilot releases the back pressure he was holding for the flare and the nose wheel comes down nicely.

Pitch trim then resets to zero.

YouTube - American Airlines b757-200 landing in Toncontin , MHTG

Its the upwind engine that is reduced.

You'll need to keep the power in through the landing to hold the attitude. This scenario is basically flying the plane onto the runway. If you were to pull the power off with a big flair, you will drift and be in trouble.

Remember, this is done at the last minute. Until then you are flying the crab all the way down. The maneuver is a transition from the crab, into the dipped wing/asymmetrical thrust, into the landing.

Not an issue in the B1900 in this scenario.


My impression is that you flew KD10s in the military and then went to a major airline. The B1900 is a whole different animal. I can understand why it sounds crazy and does not make much sense. Once you fly one, you'll understand.

Just to be clear, the technique is to lead the power reduction with the upwind engine as you apply opposite rudder so that when the mains touch, the down wind engine is carry more torque? Typically what is the torque spit?

It is my understanding that the asymmetrical-thrust crosswind technique is useful for reducing the amount of rudder required to keep the longitudinal axis aligned with the runway. (You still need to bank to correct for any drift.) If you have the rudder effectiveness available, I don't see how using this technique is practically useful (unless, of course, you're landing in a situation where the rudder is not able to keep the aircraft aligned, but if that were true I bet the bank required to correct for drift would be dangerously excessive). Either with asymmetrical-thrust or the forward-slip, you're still slipping the aircraft. Assuming this, then, why is it even considered as pragmatically useful?

In a large jet........NO NO NO NO NO!!!!!!!

I cant even believe some of the stuff I am reading here.......WOW!!!
If the xwind is that bad to make you think the only way to get the aircraft on the ground is to use assymetrical thrust it is definately time to talk about going somewhere else. I have flown biz jets as small as a citation, and airliners up to an A320, and several sizes in between, and I have never even considered this technique. As some have posted, the best, most comfortable, and safest method is to hold the crab to the flare then kick it out just before touchdown.

Yes, the downwind engine will carry more torque. Also, as the mains are touching and you know the aircraft is in control and driving onto the runway, you even out the power and reduce.

The torque split is dependent on the wind. I found that if the required torque differential was significant, that the adverse effects of said maneuver were not worth the rewards. It was more of an addition to the crosswind landing than the majority actions of the crosswind landing.

As in my case, we are talking about a B1900. Most of my flying was done in Alaska, in winter and in extreme conditions. I'm not referring to big shiny jets on dry runways.

Was there any consideratin during a go-around if you had asymetrical thrust...never, ever heard of this technique...

This is the last I am going to speak on the subject, we have pounded it into the ground.

Yes, those considerations were made. This maneuver is done at the last minute as a transition from crab to landing. The decision to land has already been made. Also, as I stated earlier, if the asymmetrical thrust was very significant, then the maneuver was not made. That in mind, if using the maneuver and a go-around was necessary, spool up was not an issue.

I understand you have never heard of this technique but not all of us at the majors came from the military or an RJ. There are a few of us who had to learn how to fly on short, icy runways where the wind blows hard. We had to have many tools in our bag and some were not in the manual.

Thats the way to do it: just before t-down reduce pwr on the dwnd engine.

I'm seeing people on here agreeing with each other, but saying the opposite things. Just as a refresher, the aircraft tries to yaw into the wind on takeoff. Therefore it is always the upwind engine that you would want to carry more power on. The only time I can think of that you would want to use this technique on takeoff would be on a multi engine TAILWHEEL aircraft, and then only until you are fast enough for rudder effectiveness.

Joe

use the force luke...

I'm still trying to grasp with why you would want to do this in the first place. I've flown the B1900 in Alaska, but not on ice. I've also landed the aircraft at its maximum certified crosswind, in the sim and in the actual aircraft.

I do not see how pulling the upwind engine first (which would increase your yaw into the wind, thus increasing your crab) helps you? You are pulling the critical engine.

If someone wants to show me a force vector diagram so I can understand the technique, I would appreciate it.

Downwind engine. It just adds a little more rudder. I've only done it in aircraft that had a crosswind limit, as opposed to a demonstrated xwind component.

It's done in the flare and has the same effect as applying rudder. It really does help on aircraft that have limited rudder authority.

Pulling the upwind engine would, as you said, make things worse.

BTW, when I said I didn't like the slip, I meant - I don't like the wing down method. When you make the crosswind correction at 500+ feet. Much prefer to do it in the flare from both a performance and pax comfort standpoint.

What cracks me up about this thread is that people are saying to use the upwind engine and and downwind engine. So there is confusion among the people who think this technique is acceptable.

Lets recap.

If you pull the downwind engine, the aircraft yaws more downwind, and increasing the drift and increasing the side loading on the gear.

If you pull the upwind engine, the aircraft yaws more into the wind increasing the crab angle, which by default increases the side loading.

If you do this in the flare, there's probably not enough time to cause any significant increase in drift or crab. However, I don't see how this benefits you either because I don't see the difference in torque being helpful.

And for the guys flying the B1900, the one big lesson that was taught to me, and I learned myself the hardway, pulling the throttles all the way to idle while in the flare was very risky. The props never came off the govenors at the same time. That gets your attention very quickly.

I agree, there's no reason to apply the forward slip very early. Typically you will see pilots who aren't very good at crosswind landings puttng the controls in early. This just adds drag and causes a lot more corrections to be made that really aren't necessary.

Think of it as more rudder authority. It's a lot like rudder in the flare. Just a means of aligning the centerline of the aircraft with the runway. I have never used this technique with any turbojet but have with airplanes that are limited, so to speak...