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I noticed many planes, especially Boeings (correct me if I'm wrong anywhere), start engine "2" first and shut down engine "2" last.

However, I was watching an Airbus A350 cockpit video when I realised the A350 starts engine 1 first and shuts down engine 1 first. Hence, I have 2 questions:

  1. In which order should the engines of the A350 be started and shut down?
  2. What determines this order and are they the same/ similar on aircraft of the same manufacturer or can they differ within the same manufacturer as well?
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    $\begingroup$ Related: Why is engine No.2 started first, instead of No.1? $\endgroup$ – Bianfable Oct 27 '19 at 9:19
  • $\begingroup$ Depends on the operating procedure of the airplane... For example the primary brake system's hydraulic pressure could be connected to the nr 1 or nr 2 engine so depending on the aircraft one of the engines should be started first but which one it is depends on the aircraft type and systems being connected to that side. $\endgroup$ – Jan Oct 27 '19 at 14:38
  • $\begingroup$ @Jan At least on a Boeing, all hydraulic systems are pressurized (electrically or pneumatically) before starting any engine. Maybe it is different for the A350 though... $\endgroup$ – Bianfable Oct 27 '19 at 14:52
  • $\begingroup$ At least in the A320 the hydraulics can be unpressurized before engine start. Even though the A320 has two electric hydraulic pumps, one for the yellow and one for the blue system - the yellow pump is only running when the cargo doors are operated or when set on manually from the flight deck and the blue pump only kicks in when the first engine is started (or in flight as a backup). At least as far as I know... I'm only a simulator pilot not a real world airline pilot. $\endgroup$ – Jan Oct 27 '19 at 14:56
  • $\begingroup$ A good answer may include a diagram of hydraulics (or a link to it) for all the mentioned aircrafts (B737, A350, A320) highlighting the differences in pressurising means for all circuits. $\endgroup$ – Manu H Oct 28 '19 at 9:45
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For some general reasons, see Why is engine No.2 started first, instead of No.1?

Airbus A350

Specifically to the Airbus A350, I could not find any reason why one engine must be started first. As far as I know every modern airliner could start engines in arbitrary order.

The Airbus A350 has two hydraulic systems. From the Airbus A350-900 Flight Deck and Systems Briefing:

The A350 hydraulic system has two independent hydraulic circuits:

  • The YELLOW hydraulic circuit
  • The GREEN hydraulic circuit

Each system can be pressurized with an engine driven pump from either engine, or electrically on the ground:

Engine Driven Pumps (EDPs)

Four EDPs pressurize the hydraulic system. On each engine there are two EDPs, one pressurizes the GREEN hydraulic circuit and the other pressurizes the YELLOW hydraulic circuit. Thus if an engine fails, the remaining engine can still pressurize both hydraulic circuits.

Electric Motor Pump (EMP)

One EMP per hydraulic system can provide hydraulic pressure on ground only, when all engines are shut down. For example, the EMP of the YELLOW circuit operates automatically for cargo door actuation.

The main wheel brakes are powered by both hydraulic systems:

A350 Brakes

And also the parking brake uses pressure from both accumulators:

A350 Brake Accumulator

Which engine is started first has therefore no impact on which hydraulic system is powered first or which type of braking is available.

Airbus A320

The situation is a bit different for the Airbus A320. As Jan pointed out in the comments, it is normal to start the right engine first, because it provides hydraulic pressure to the yellow system. From the A320 FCOM Hydraulic chapter:

Green System Pump

A pump driven by engine 1 pressurizes the green system.

Blue System Pumps

An electric pump pressurizes the blue system. A pump driven by a ram air turbine (RAT) pressurizes this system in an emergency.

Yellow System Pumps

A pump driven by engine 2 pressurizes the yellow system. An electric pump can also pressurize the yellow system, which allows yellow hydraulics to be used on the ground when the engines are stopped. Crew member can also use a hand pump to pressurize the yellow system in order to operate the cargo doors when no electrical power is available.

The yellow system powers the alternate/parking brake system:

A320 Hydraulics

However, starting the left engine first is still perfectly possible. Hydraulic pressure from the accumulator is still available after pressurizing the yellow system with the electric pump before pushback to provide braking.

Boeing 737

The Boeing 737 has two hydraulic systems: A and B. Both can be powered by electric pumps. From the FCOMv2 13.20.2 Hydraulics - System Description:

Both A and B hydraulic systems have an engine–driven pump and an AC electric motor–driven pump. The system A engine–driven pump is powered by the No. 1 engine and the system B engine–driven pump is powered by the No. 2 engine. An engine–driven hydraulic pump supplies approximately 4 times the fluid volume of the related electric motor–driven hydraulic pump.

Boeing 737 Hydraulics System

The braking is provided by both systems. From the FCOMv2 14.20.3 Landing Gear - System Description:

Normal Brake System

The normal brake system is powered by hydraulic system B.

Alternate Brake System

The alternate brake system is powered by hydraulic system A. If hydraulic system B is low or fails, hydraulic system A automatically supplies pressure to the alternate brake system.

Brake Accumulator

The brake accumulator is pressurized by hydraulic system B. If both normal and alternate brake system pressure is lost, trapped hydraulic pressure in the brake accumulator can still provide several braking applications or parking brake application.

Again, there is no reason to start one engine before the other because of hydraulics/braking.

Conclusion

Engine start order is mostly tradition and will be specified by the airline op-specs. The shutdown oder also does not matter since both engines are usually shut down at (almost) the same time at the gate, except for a single engine taxi where the hydraulic system that powers the nose-wheel steering must be pressurized.

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    $\begingroup$ Interesting thing is, that if the A320 family is taxiing with just one engine, it uses #1 because only then it has brakes and nose wheel steering (except the PTU is used, which is usually avoided, afaik). See this answer $\endgroup$ – PerlDuck Oct 28 '19 at 18:38
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Boeing 787 is usually started Right then Left but the start process for both engines is concurrent. So the Commander calls START RIGHT, then opens the right fuel control switch, then immediately calls START LEFT and opens the left fuel control switch.

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