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Controlling The Throttles Manually On Classic B747  
User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 20
Posted (14 years 2 days 4 hours ago) and read 3022 times:

Hi everyone,

Those who have piloted the classic B747s before, would like to know if the aircraft is difficult to handle when on manual control of the throttles since the engines produce different thrust each?



Boeing747 万岁!
16 replies: All unread, jump to last
User currently offlineTuffty From United Kingdom, joined Mar 2001, 92 posts, RR: 0
Reply 1, posted (14 years 2 days 2 hours ago) and read 2995 times:

Though im not a pilot ive been told by some flight engeneers who served on the 200's that they would stager the throttles to get all of the engines to have te same power out put. however i dont think that this was what you were asking and i to am interested to know if they were awkward to fly with the throttle stagger taken out, say at a change of throttle possition on approach.
regds tuffty  Smile

User currently offlineMandala499 From Indonesia, joined Aug 2001, 7562 posts, RR: 76
Reply 2, posted (14 years 1 day 14 hours ago) and read 2940 times:

if it staggers, won't they just rudder trim or aileron trim the plane ?


When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 20
Reply 3, posted (14 years 1 day 11 hours ago) and read 2920 times:

Rudder trim? The B747 has four engines though. How is it to help? Any B747 classic pilots?

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User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 4, posted (14 years 1 day 10 hours ago) and read 2908 times:

Mandala think about it.

Q: Why do they stagger the throttles?
A: To have the engines producing the same output more or less.

Q: What would happen if they didn't stagger the throttles?
A: Each engine would produce a different amount of thrust.

Q: What side effects if any would arise?
A: Each engine producing a different amount of thrust, each engine having a different fuel consumption.

Q: What are the benefits of staggering the throttles?
A: Like mentioned previously engines producing same amoung of thrust more or less, all gauges on the panel reading the same more or less, thus making it easier to see if there is a problem with one engine as it's corresponding gauge would be out of line with the others.

Q: What does rudder trim do then?


User currently offlineMandala499 From Indonesia, joined Aug 2001, 7562 posts, RR: 76
Reply 5, posted (14 years 1 day 9 hours ago) and read 2896 times:

My question is what happens when the pilot hand throttles the plane if staggering is required... ??? how do they deal with it ? I was guessing rudder trim...


When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineBR715-A1-30 From , joined Dec 1969, posts, RR:
Reply 6, posted (14 years 1 day 6 hours ago) and read 2880 times:

Why do they have to stagger the throttles. What I mean is why do the engines produce different amounts of thrust if you dont stagger them. Why aren't they like the 717 where both throttles in the same place equals both engines at the same thrust.

View Large View Medium
Click here for bigger photo!

Photo © Dean Barnes

Note how the throttles are staggered in this picture.

User currently offlineBarney captain From United States of America, joined Nov 2001, 1473 posts, RR: 13
Reply 7, posted (14 years 1 day 3 hours ago) and read 2865 times:

In a perfect world, they would all be in alignment. From time to time, it's not at all surprising to find a little out-of-rig situation. Typically, you just match the N1 to produce symmetrical thrust. When hand flying during an approach, I usually match fuel flow and yes, some times there is a little throttle split but it is usually easily managed.

Southeast Of Disorder
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 8, posted (14 years 1 day 2 hours ago) and read 2850 times:

The amount of discrepancy of mis-rigged throttles has a limitation... I think it's one knob width from any other throttles.

When handling the throttles while manouvering as in an approach situation the throttles are moved as a group. When the engines stabilize I will look down at my EPR and make a quick adjustment.

The amount of throttle mis-rigging permited doesn't allow for large errors in engine trim.

I have written up throttles with large discrepancies beyond limitations.

During most of the flight the engines will be trimmed by the flight engineer after the captain makes the initial power adjustment. ex. cruise to climb power.

The only time they are not trimmed by the engineer are in manouvering situations.

Rudder trim in never used in this situation.


User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 9, posted (14 years 19 hours ago) and read 2817 times:

The limits we use in our MM are:
Up to 1 knob from difference, no action required
1 to 1.5 knobs defered for 5 days for trim run and correction
Over 1.5 knobs must be fixed before further flight

A throttle on a classic can be out of rig for several reasons, but the miost common we come across is a slowly wearing jet fuel control. This normally requires trimming the fuel control under controlled circumstances until the throttles match or the fuel control is replaced because the trim screw is bottomed out.

User currently offlineMandala499 From Indonesia, joined Aug 2001, 7562 posts, RR: 76
Reply 10, posted (14 years 11 hours ago) and read 2788 times:

OK.. thanks for the answers.

4 engines in staggered poses an interesting question. How about the AN225 and B52 pilots?  Smile


When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineVC-10 From United Kingdom, joined Oct 1999, 3727 posts, RR: 32
Reply 11, posted (14 years 11 hours ago) and read 2792 times:
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On a JT9 another cause of throttle stagger is the Engine Vane Control going off schedule. Our Co policy was to carry out an EVC trim every year to bring the EVC back on schedule.

To expand on Air2gxs's comment, as the the JFC wears it tends to lean off the fuel.

Another cause on all engine types is bleed air leaks or internal engine damage.

User currently offlineExpratt From United States of America, joined Mar 2000, 311 posts, RR: 0
Reply 12, posted (14 years 2 hours ago) and read 2762 times:

No two engines will ever have the same performance. Not even two new production engines in serial number sequence will have the same performance. Most of the time, the differences in performance between engines is fairly slight so there is not much if any change in throttle position. The bigger the engine like a JT9D, the more likely it will show throttle stagger than it will on a small engine like the BR715. In addition to the items previously listed, another cause for throttle stagger is engine deterioration. As the engines age, the throttles have to pushed higher to attain a given amount of thrust. With engines of similar vintage installed, the added throttle push is not noticeable because the engines will most likely age the same and the throttles will be pushed forward similar amounts. If one of the older engines is replaced, the throttle for the newer engine will not have to be pushed as high as that on the older engine to attain the given amount of thrust, hence the throttles become staggered and difference becomes obvious. Engine deterioration can be many things: worn compressor and turbine outer air seals, bowed or burned turbine nozzles, dirty compressors. As was previously stated, there are limits on how far the throttles can be staggered. Most allow up ot one knob width before maintenance has to adjust the throttle rigging or engine trim.

User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 13, posted (13 years 12 months 4 days 23 hours ago) and read 2755 times:

The reason for 747-400 and more modern aircraft not having the need of staggered their throttles, is that they have FADEC (Full Authority Digital Engine Control) which is the fly-by-wire of engines. Depending on throttle position, the system controls the adequate fuel flow electronically. No direct mechanical links from the throttle levers. This is more reliable as to precission, and that's how the same thrust (more or less) can be generated on all four engines without the staggering.


User currently offlineTuffty From United Kingdom, joined Mar 2001, 92 posts, RR: 0
Reply 14, posted (13 years 12 months 4 days 21 hours ago) and read 2743 times:

yeah thats right on the 400's yove got a trim plug on the fadec that denotes the max thrust rating making it an "h" or "g" rated engine. this will also allow the engines performance to be flat rated so that at ISA conditions you are given an exact thrust rating which is acurate. this alllows any differences in the engines from manufacture to be alleviated.

regds tuffty  Smile

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 15, posted (13 years 11 months 4 weeks 19 hours ago) and read 2614 times:

Flying a 747 (or any other 4 engine aircraft - I flew the 707s and DC8s as well) is never a problem, power adjustments are done by "feel" and, if necessary, by asking the flight engineer to adjust EPR (on the JT9s) or N1 (on the CF6s)...
As a matter of fact, I would sometimes "stagger the power" in cross-wind landings... And flying an "ILS" in real life (or in a simulator) requiring a very slight power adjustment, I would do it, using the "inboard" engines only, such as pushing "half a knob" on both "inboards" to gain an extra 2 or 3 knots on the approach (Vref) speed... never a problem... hell, we practice with outboard engines "out", even with "two engines out" on the same side... the 747 is a very easy and stable aircraft to fly...

User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 20
Reply 16, posted (13 years 11 months 4 weeks 13 hours ago) and read 2586 times:

Thanks a lot everyone for the responses. Certainly learned a few things from this thread. Thanks!

747Skipper:Does the B747 (except -400) have autoland?



Boeing747 万岁!
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