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Inboard Engine Taxi on Four Engine Airplanes??

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Inboard Engine Taxi on Four Engine Airplanes??

Old 31st Aug 2015, 08:22
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Question Inboard Engine Taxi on Four Engine Airplanes??

Gents (and ladies), please help me understand…
Over the years I’ve often observed four-engine turboprops taxiing to and fro with only their inboard engines turning. Jets do so as well, but it is more difficult to observe which engines are powered and which are not. I recognize that that two engines will provide adequate power for taxiing, and that this is generally a fuel conservation choice. What I do not understand is why ground operations typically use the inboard engines, usually numbered two and three, with one and four shut down. It seem to me that using number one and four would give the pilot slightly better control of the airplane through the use of differential thrust, when necessary, perhaps including some reduced wear on tires and brakes. While not all four engine aircraft taxi on two, all the time, I see it often enough to note that the two turning are nearly always the inboard engines. Can someone please explain why 2 and 3 are usually chosen, as opposed to 1 and 4? So far, the only viable reason that I can come up with suggests that I may have something to do with either hydraulic systems (brakes & steering) or electrical generators. If it helps, I’ve seen this most often with Electras and C-130 Hercs. Thank you in advance. There must be a reason...
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Old 31st Aug 2015, 08:33
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It is decided by the manufacturer. Engines 1-4 are the green system on the Airbus 340, and from this, normal brakes work and nose wheel steering, but not parking brake, this is engine 2 blue system, which is not running.

Outboard engines also provide more torque for a turn, making a tighter turn a little easier with less thrust when compared to an inboard engine, I suppose.

A330 has a green hydraulic system working with both engines 1-2, so either maybe shut down.

B777, either engine can be shut down and the same for my B787-9. Either engine.

Many conditions are to be considered before single engine taxi, and supplementary techniques and checklists need to be followed. You must think ahead and not all airports allow "reduced engine taxi", is the technical term!
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Old 31st Aug 2015, 08:35
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Less likely to ingest FOD through 2+3 as 1+4 often overhang the grass etc? Just a generic starter for 10, I'm sure there's plenty of other reasons depending on the aeroplane too.
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Old 31st Aug 2015, 09:46
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Just make sure you know your power sources to the brakes. These maintenance guys tried inboard engine taxiing of a 747 and look what happened.

AirDisaster.Com: Accident Photo: Saudi Arabian Boeing 747-368 (KUL) (4)

Leaving the outboards running can make taxiing easier when using differential power. Herc was a good example. No tiller required except for the tight turns in the parking area with inboards shut down.
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Old 31st Aug 2015, 09:51
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I always taxi on the inboards in my 767.
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Old 31st Aug 2015, 10:46
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A330
Preferable to shutdown the right Engine as the Left powers both Green and Blue HYD. Giving you normal AND Altn brakes (park brake accumulator etc )

That's what we are "told" to do anyway.....
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Old 31st Aug 2015, 17:08
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On the larger 4 engine aircraft (747 and A380), using the outboard engines above idle during taxi can result in nasty consequences to those behind as it throws out dirt, gravel, etc. from the edges of the taxiway. This is especially true of airports that don't see a lot of 747 operations to keep things 'swept'
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Old 1st Sep 2015, 11:18
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Thank you

And the winner is... Preventing FOD!
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Old 1st Sep 2015, 11:36
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On the larger 4 engine aircraft (747 and A380), using the outboard engines above idle during taxi can result in nasty consequences to those behind as it throws out dirt, gravel, etc. from the edges of the taxiway.
Aren't the outboard engines higher off the ground than the inboards?
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Old 1st Sep 2015, 11:58
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146/RJ often use inboards as the APU is used for landing air so it provides electrical while inboards have hydraulic duties. (Genies are on the outboards on the Jumbalino....)
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Old 1st Sep 2015, 15:05
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Originally Posted by Capt Fathom
Aren't the outboard engines higher off the ground than the inboards?
Yes, but the jet wake still contacts the ground just passed where the tail ends. On some taxiways the outboard engines hang off the paved areas so there is a lot of stuff that can be kicked up.

A few years ago we were doing some flight testing of a 747-8 at the Boise, Idaho airport (that doesn't normally see many 747s). We actually got a panic call from the tower to not use the outboard engines during taxi - apparently we were throwing up quite the cloud
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Old 1st Sep 2015, 15:37
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tdracer,
On the larger 4 engine aircraft (747 and A380), using the outboard engines above idle during taxi can result in nasty consequences to those behind as it throws out dirt, gravel, etc. from the edges of the taxiway.
Not only that, the outboard engines that overhang taxiways can also suck up both small and larger particles depending on the degree of throttle advances above idle. Both can damage engine components.
The first DC-8 aircraft which were re-engined with CFM56 engines began to show HPT blade distress (burned blade tips) very early in their lives on outboard engines. It was found that fine sand was clogging the turbine blade cooling passages at the blade tips. The fine sand was traced to taxiways at KPHX. Even although there were air-particle filters in the engines to prevent contamination, this particular unique sand consistency made it through and ultimately caused the tip burnout problem, Murphy's Law at work. The solution was to enlarge the blade tip dust holes which corrected the problem.
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Old 1st Sep 2015, 17:43
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Originally Posted by Turkish Delight
146/RJ often use inboards as the APU is used for landing air so it provides electrical while inboards have hydraulic duties. (Genies are on the outboards on the Jumbalino....)
Beware the effect on the hydraulics if the APU should fail on the ground however (not at all uncommon in hot weather.)
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Old 1st Sep 2015, 18:08
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Indeed. The old 146 used to trundle around quite nicely on 3 anyway.
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Old 1st Sep 2015, 19:26
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Old 1st Sep 2015, 21:14
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On the 744 we don't shut down #1 because it's hydraulic system powers the nose and body gear steering, and we don't shut down #4 because it's hydraulic system is primarily resposible for braking.

Yes, there are backup pumps to pressurize these systems in case of an engine not running, but it's better just to shut down 2 and/or 3.
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Old 2nd Sep 2015, 02:55
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Don't forget, also, that the inboards are much lower on the 747, so FOD potential is much higher with them when spooled up.
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Old 4th Sep 2015, 01:46
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Hi,

Turbine D,
Can you please talk about these air particle filters (mentioned in your post) in the engine for turbine blades cooling.

Feedback appreciated.
Regards
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Old 5th Sep 2015, 21:48
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Originally Posted by AerocatS2A
Beware the effect on the hydraulics if the APU should fail on the ground however (not at all uncommon in hot weather.)
Your Standby Gen. driven off the Green system will retain some AC power, and will suck all your Green Hyd system services.

You always have yellow and if your brakes are selected to be running on Yellow (as they should be!), ya should always have braking. Don't need no fancy electrical at all on the 146 for Yellow system braking.

You will loose your nosewheel steering though! (eeek...Green)

Ya those bloody APUs. Only time we taxi 2-engine on the Smurf-Jet was just quick maintenance runs.
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Old 5th Sep 2015, 22:21
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Originally Posted by Turkish Delight
146/RJ often use inboards as the APU is used for landing air so it provides electrical while inboards have hydraulic duties. (Genies are on the outboards on the Jumbalino....)
Which of the 5 APUs are you referring to?
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