I recently travelled with an A320 and an A321 from FRA (Frankfurt) to TXL (Berlin) and back again. On this and on previous flights I already noticed a whining sound, much louder than other sounds, while we were still at the gate or soon after leaving the gate and starting taxiing. I cannot remember exactly. My research showed that this was the PTU's sound (aka “barking dog”), typical for an A320 and its sisters.

One of my references was this post. The author said:

[The PTU can be heard]
At the gate, especially if we taxi in on one engine. During the taxi, the left engine is running and pressurizing the green system.

This led me to this question:

When taxiing with one only engine, do pilots always use the left-most one (#1)?
Or do they randomly choose one of the two (or four)? If they always use the port/port outer engine, doesn't it wear out sooner? Or are the engines swapped at regular intervals (like car tyres are swapped from front to rear)?

  • 3
    $\begingroup$ OpSpecs may prohibit the use of one engine on taxi. I worked for an operator who required two engine taxi to reduce (waht they thought was) excessive and needless sideloading of the nosewheel. This operator has also killed a number of pilots and has a depressingly bad accident/incident rate, so take their behavior with a grain of salt. $\endgroup$
    – acpilot
    Jan 9, 2017 at 2:43
  • 8
    $\begingroup$ Not an answer, but notice that wearing both engines equally might not be desirable. You don't want them to approach their end of life at the same rate, to minimize the chance of a double failure. $\endgroup$ Jan 9, 2017 at 10:02

2 Answers 2


Airbus aircraft have 3 hydraulic systems (Green, Blue, Yellow). Green is pressurized by left engine, Yellow by right engine, and each one can be pressurized by the other using the PTU. In addition the Yellow circuit has an electric pump.

The Blue circuit is a backup pressurized on demand by several means (electric pump and RAT).

Each circuit powers different aircraft components:

enter image description here
Source: A320 FCOM

The choice of which engine(s) to use is based on the need to have pressure:

  • For brakes
  • For nose wheel steering system.

Normal brakes and nose wheel steering are pressurized by the Green system. The brake accumulator must be pressurized, this can be done by running the an electric pump without starting an engine..

Engines are numbered 1 for left, 2 for right on twin-engined aircraft and from left to right (1/2 and then 3/4) for four-engined aircraft.

From this document:

Which Engine to Use?

A320 Family: ENG 1
A330: ENG 1
A340: Outer engines (ENG 1 + 4)


A320 Family:

  • ENG 1 pressurizes GREEN HYD (NWS + NORM BRAKE)
  • YELLOW HYD pressurized via ELEC PUMP
  • PTU is not needed


  • ENG 1 pressurizes GREEN + BLUE HYD
  • ELEC PUMP are not needed

A340 (outer engines when possible -- exception would be a narrow taxiway and FOD risk):

  • ENG 1 + 4 pressurize GREEN HYD (NWS + NORM BRAKE)
  • Check ACCU PRESS normal before ENG start
  • ELEC PUMP are not needed
  • 3
    $\begingroup$ Great answer. Especially the Which and Why sections. I'm still curious about the wear of ENG 1 compared to ENG 2 when ENG 1 is always favoured but I'll post another question on that. Perhaps. ;-) $\endgroup$
    – PerlDuck
    Jan 8, 2017 at 19:30
  • 3
    $\begingroup$ If ENG 2 would be used (on the A320), it would pressurize the Yellow system, but not the Green one. So normal breaks and steering system would not be active unless the PTU was used. To prevent the PTU to be active, this is done with ENG 1 (and only the electric pump of the Yellow circuit if the brake accumulator is not pressurized enough -- no PTU, no right engine). $\endgroup$
    – mins
    Jan 8, 2017 at 19:35
  • 2
    $\begingroup$ So the intention is to not use the PTU deliberately. Understood. NWS either works with ENG1 or ENG2+PTU. Given that, using ENG1 is the simpler choice. $\endgroup$
    – PerlDuck
    Jan 8, 2017 at 19:55
  • 2
    $\begingroup$ @PerlDuck: Right, as soon as there is a difference of pressure between Green and Yellow of more than 500 PSI. Here is the full logic from the FCOM. $\endgroup$
    – mins
    Jan 8, 2017 at 20:49
  • 3
    $\begingroup$ @PerlDuck, the PTU is an emergency-only thing. It tends to heat up rather quickly, so you don't want to use it during taxi so it's not already hot in case you needed it due to engine problem during take-off. $\endgroup$
    – Jan Hudec
    Jan 9, 2017 at 18:11

I can't say if pilots always use the same one engine for taxi in or out. However, if they do, that engine probably lasts longer and the other one wears out sooner. The reason is that engine life is highly dependent on low cycle fatigue due to thermal cycles (i.e. cold then hot then cold). If you start a cold engine and immediately go to takeoff, the thermal transients inside are more severe than if you let it warm up at idle for 10 minutes. Same on landing. Shutting the engine down immediately after landing is more severe than cooling off at idle for a few minutes.

  • 3
    $\begingroup$ There are aircraft limitations for warm and cold startup a and cool down time limits to prevent wear like this. $\endgroup$
    – casey
    Jan 8, 2017 at 17:52
  • 4
    $\begingroup$ This is not true for commercial jet engines. There are limits to how long the engines must run for after start before take-off depending on the engine types, SOPs and how long the engine has been shut down for. $\endgroup$
    – Simon
    Jan 8, 2017 at 17:54
  • $\begingroup$ Yes, there are limits on minimum idle time before takeoff and after landing. It's often 3 minutes. If you go more than the minimum, the LCF life would be extended. Maybe not by much, but it would be better. The engine is not at thermal steady state after 3 minutes. $\endgroup$
    – Daniel K
    Jan 8, 2017 at 23:42
  • 3
    $\begingroup$ My main point was just that taxing out on one engine definitely does not "wear out" that engine. $\endgroup$
    – Daniel K
    Jan 8, 2017 at 23:47

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