Hi all

I have been wondering if the engines on big jets such as a 737-800 are always throttled together in sync or is there any occasion (other than a malfunction or failure) when different throttle settings may be used for each engine, such as quicker turns or cross wind landings etc?

1 hour ago, chickster25 said:

such as quicker turns or cross wind landings etc

Differential thrust would initially increase the yaw and you can achieve that by using rudder basically. I gather it would be a very ineffective way (and slow) to make the turn sharper in the air.

Turning is done via horizontal lift component (roll/bank).

On the ground however, differential thrust is used on tight corners.

Other than on the ground (as @SAS443notes above), you don't want asymmetric thrust in a multi-engine airplane, big or small. It's going to give you sideslip, which you usually don't want. On the occasions when you do want sideslip, e.g. in a crosswind landing, the rudder is going to give you the same effect and will allow much more precise control.

If you want to tighten a turn, you should increase the angle of bank. Using yaw to tighten a turn, whether through asymmetric thrust or the rudder, will make the turn uncoordinated, which is aerodynamically inefficient, uncomfortable for passengers, and can cause a spin if you're slow. 

Edited August 9, 20223 yr by martinboehme
Reference SAS443

So as i have only one lever on my throttle to control both engines, I am not really missing much. I had thought about removing the airbrake from the 3rd lever and reassigning that for the second engine, but other than some extra ground control, it doesnt seem like Im really going to be using it for much?

That's correct. The only case where you'd really need to control the two throttles independently is if you have an engine failure. 

On older twin jets there could be an annoying resonance which occurred at cruise settings. I still notice it, to a lesser extent, when travelling as a passenger on modern aircraft. We used to deal with it by making very small variations in the power settings between the two engines, as little as 1%. It sometimes took a bit of experimenting to find the sweet spot, without affecting lateral trim, but it could reduce the resonance quite noticeably.

the older Boeings have one Autothrottle Clutch which drives the Trust Levers together at one axis, the newer like B777 and B787 has seperate AT Drives for each Thrust Lever. Best regards Ralf

On some of the older 3/4-engine jets, it was common practice to pull the outboards all the way back, while keeping the inboard(s) up higher for improved bleed air flow.  In the C-141 we called that configuration "MAC idle"--it helped keep the pressurization stable during descent and was especially important if you were descending into icing and anticipated a need for the high bleed air load posed by wing anti-ice.  Also, on large 4-engine jets, it's common practice to keep the outboards back at idle during taxi when they are hanging out over an unpaved surface or a FOD-strewn shoulder.

In general, the thrust levers are moved together. Most of the business jets have engine sync features/switches that sync the N1 or N2 spools, depending on which selection provides the less noise and vibration.  Typically, it's the N1 spool, but occasionally you find N2 works better.  In the larger airplanes, that is something likely incorporated into the autothrust/autothrottle system.  In the days before engine sync systems, you would find thrust lever setting and EPR or N1 setting that would keep the engines in sync and keep the cabin quiet during cruise.  

If you watch some of the YouTube videos of the older Boeing 747s and the like in cruise flight, it's not uncommon to see thrust levers at different positions as the FE tries to match N1 or EPR settings for cruise or to keep the engines in sync.  Each engine will develop its own personality overtime, and together will make that airframe different from airframes in the fleet in terms of fuel burn.  We have one Challenger 300 that burns about 100 lbs/hr more than the others because on engine is burning a bit more fuel than the other.  If we match the engines on fuel flow, the N1 setting would not be in sync.

When I used to fly the TFE-731 Lockheed Jetstar years ago, it was common to just use the inboard engines at slower speeds, for example in the pattern or approaching the airport.  At 200 KIAS, the inboards would provide enough thrust to maintain level flight. The outboard engines would be set to idle thrust.  You didn't bring the outboard engines back up until you began to configure for landing by lowering the gear and extending landing flaps. 

 As far as using asymmetric thrust for taxiing, generally you don't have to do that in a jet; however, never say never.  There have been sometimes when I have pushed the outboard engine up slightly during a turn, usually on a taxiway where I have to make 180 turn.  Seldom do it on the ramp because of the safety issues involved with increased thrust around the ramp area.   

As far as crosswind landings go, I believe the general consensus is that it would be considered bad technique to do so.  On most jets, you want to close the thrust levers starting at about 30 feet RA.  Some airplanes, you might slowly start reducing so that they are at idle at touchdown (e.g., 20 series Lear, DC-9) as these airplanes have tendency to fall out of the sky if thrust is reduced too fast. Boeing recommends this too, for their airplanes.  Other airplanes, you can bring the thrust levers to idle at 30 feet and airplane will settle nicely to the runway, e.g., the Challenger 300 with its big glider like wing.  In any event, you don't want to introduce asymmetrical thrust in the flare.  That's a good way to start going for a ride.  Timed right, a steady push of the rudder to align the nose with the centerline is all that is needed in a crosswind, along with a slight wing down into the wind if the flare extends to prevent drift - depending on OEM recommendations.  Some do not recommend putting a wing down (e.g., Dassault Falcon). 

Rich Boll

5 minutes ago, Bob Scott said:

On some of the older 3/4-engine jets, it was common practice to pull the outboards all the way back, while keeping the inboard(s) up higher for improved bleed air flow.  In the C-141 we called that configuration "MAC idle"--it helped keep the pressurization stable during descent and was especially important if you were descending into icing and anticipated a need for the high bleed air load posed by wing anti-ice.  Also, on large 4-engine jets, it's common practice to keep the outboards back at idle during taxi when they are hanging out over an unpaved surface or a FOD-strewn shoulder.

Bob...We're showing our age! 

Edited August 9, 20223 yr by richjb2
typo correction...