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I have read that pilots only taxi on 1 engine to save fuel. Which engine does a twin engine jet such as a 737, 777, a320 or similar use to taxi? Left or right, or does it vary by pilot or some other factor?

On 4 engine jets such as a 747 or a380 pilots use 2 engines. I assume its a matching pair, inner or outer. But again, which pair?

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    $\begingroup$ It's completely up to the airline, what engine and how many are used to taxi. $\endgroup$ – Noah Krasser Jul 21 '17 at 6:01
  • $\begingroup$ Possible duplicate: Taxiing with one engine: Is engine #1 always used or do they switch? $\endgroup$ – mins Jul 21 '17 at 6:45
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    $\begingroup$ @mins out of interest, if you found the duplicate why didnt you enter a close vote? $\endgroup$ – Jamiec Jul 21 '17 at 9:30
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    $\begingroup$ @Jamiec: The OP could be interested in Embraer, Boeing, Bombardier, in addition of Airbus and its specific PTU. $\endgroup$ – mins Jul 21 '17 at 11:07
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In my experience it is not "common" for air carrier aircraft to taxi with an engine shut down. It does happen of course, but there is generally a specific reason behind that procedure if used.

Reasons may be:

  1. Fuel load is close to the minimum required for the flight at the start of taxi (a variety of reasons for this).

  2. Long line up on the taxiway awaiting takeoff clearance.

  3. Captain discretion to save fuel.

For most Air Carriers, the engines are started using air pressure from an APU (auxiliary power unit) affixed to the fuselage (at the tail of the aircraft). The start procedures are initiated while the aircraft is being pushed back from the gate by a tug. Shortly after the aircraft is pushed back and generally aligned for taxi both engines (twin engine aircraft in this example) have been started and an "after start" (or similar) checklist has been completed. Besides the engine noise, which is sometimes quite low and hard to hear, the end of the start procedure can be noticed when the lights blink in the cabin as the electrical power generated by the APU is being replaced by the engine driven generators. Also, there is generally an increased airflow into the cabin as air from the engines is vented to the cabin by the air conditioning system. Very little air is available during engine start from the APU because the air is being used to start the engines.

Anyway, this is probably a more expansive explanation than necessary for your question (I just wanted to give some context to the process of engine start for air carrier jet aircraft).

If the aircraft is taxied out using a single engine, the crew can either start the remaining engine at some point prior to takeoff using the APU (if left operating) or by cross-bleeding the engine air from the running engine to the engine now being started.

If the aircraft is taxied with one of its engines not started at the gate, the crew has to run an additional checklist during the taxi as and after the engine is started. Not a big deal, but a bit more workload.

Lastly, there have been occasions where problems have resulted from taxiing with one of the engines not started. For example, many years ago a B727 (3 engine aircraft) started its takeoff roll with only two engines running because the crew failed to start the third engine. It crashed on takeoff. Also, there have been occasions where the aircraft was not configured properly for take off (e.g., engine bleed switches in the wrong position) due to a failure to properly follow/run the appropriate checklist for starting an engine during taxi or configuring the bleed/pressurization system properly for take off.

Again, taxiing with an engine not yet started does happen. But considering how many flights occur every day in the world it would be difficult to say this procedure would be anything close to common.

Just my two-cents

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I believe that it depends on the airline's procedure whether or not to shut down an engine for taxi.

I think 4 engine jets use one on each side but for 2 engine jets the engine chosen has no impact and thus the one chosen is not very relevant.

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    $\begingroup$ The chosen one is very relevant since each engine powers different hydraulic circuits. $\endgroup$ – DeltaLima Jul 21 '17 at 7:35
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Interesting historical note to the answers already listed is that the Boeing 707 could only shut down #2, #3 and #4 engines for whatever purpose without availability of a ground air-start unit (huffer). The #1 engine nacelle has a "bump" on top which is the compressor to start the other engines. If ground-start air wasn't available then the #1 engine had to be kept running.

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