Most operators of 747s I have worked with its 4 1 2 3....
4 for Brakes (Also Electrical Power, ESS Bus from this side)
1 for Steering/Brakes, Also powers other side of sync bus.
2 (Brakes again!!) Proper Redundancy there eh!
Then 3

For those whose APU has enough power to start both,...is it ever done?

I picture most ops manuals being touchy about that

On the -400, dual starts is very common.

since I have not operated anything else than G.E.'s on the -400, I cannot say about other engine types and their stating sequence.

The MH B777 that I used to look after at ARN normally started both engines together. Whole process took about 30 secs from Clear to start, to Clear to disconnect.

History!
 

Flaperon777 Has most of the historic answers but a little thin on historic technical detail.

Historicly the #2 engine had the generator and the #1 had the hydrulic pump so the battery was mounted nearest to the #2 engine to enable the shortest cable run to reduce the electrical losses due to resistance.

So the #2 was the best engine to start in order to save the small amount of electrical power that these aircraft had in the batterys and once it was running to have a generator on line to help start the #1 engine.

The Beech King Air has the battery mounted in the wing just inboard of the engine, in these modern times I suspect Beech mounted the battery in this location because thats the way they had done it in the past.

With the exception of the B707 which we started in the order previously stated, most of my experience with both Boeing and Airbus was to start 1 first. I guess each operator has their own reasons. Boeing training-at least on the 777 does not have a preference for 1 or 2.

I wonder what the sequence is on the B-52. Or the An-225, for that matter.

737-3/4/5 engine start 2 first due to the fact that if the isolation valve fails you find out before you waste an engine start. Isolation valve failure is a no-go item.:ok:

On our RR's and GE's (747-400), we start 4, 3 then 1&2.

I really don't think you can claim that procedures are truly "common" until you have worked for every airline :ok:

As flyhelico
"Dear A330-340

on the 320 bus, engine 2 power yellow hydraulic for parking brake(the handle on the pedestal).

once second engine is started(left engine # 1), the brake valve switches from yellow to green, and provide hydraulic pressure for normal braking."

This is correct. Also on the A330, engine # 1 started first as the 2 Hydraulic Pumps on it (Blue and green Systems) will allow normal braking and Alternate braking upon start up immediately.

I asked same question many years ago, why #2 engine (#4 on the 747, 744 ) started first.

I was told at the time, it was for 2 reasons.
1, As previously discussed this engine drives the hydraulic pump that supplies the primary braking source,
2, The "essential" power BUS normal source also comes from this engines'
generator. ( Not quite true in the 767)

When you look at the A330 the Primary and Alternate green and blue hydraulic brake source come from Engine 1, as well as the normal Essential power supply is also from Engine 1.

Does the A330 start engine 1 first??

Jep. 330 start first ENG 1.

Historic
 

The main reason is HISTORIC.:ok:
“Back in the day” on the old piston props the crew would start No 2 while the Px were boarding on the left side door.
On turn around they would shut-down 1 to disembark and board the next load and keep 2 running. What’s more, the only reason 1 was shut down, was because it was too noisy for the Px, nothing to do with safety.
Ahhh! Those were the days!
:)
How do I know? My dad used to fly them.
;)

IMHO, right engine was choose because

- it was connect to cabin air condition pack and help passengers from getting sweat. [For ex. B737]
- Hydraulic pressure for brake.
- may be reduce the risk for ground staff since the cargo loading and gate are
done on the left side.

Best regards

The starting order always corresponds to the engine hydraulic pump that supplies the brake system.
A330 #1 = green + blue hyd pumps = normal + alternate/parking braking.
737 #2 = B sys hyd pump = normal braking.
767 #2 = R hyd system = normal braking.
744 #4 = #4 hyd system = normal braking.

If you're pushing back you can't really justify starting #1 or #2 first based on which one is farther from the jetway, but here's an important thing to think about...

Boeing used to power A hydraulics with EDPs (Engine Driven Pumps) and B hyrdaulics with EMDPs (Electric Motor Drive Pumps, also, I realize some aircraft have more systems in general and there are also AUX and STBY systems on others, but let's stick to the basic 2 system layout). This was fine until they came up with the crazy idea of only slinging 2 engines under a plane. The first 737s (100 and 200) did have this same layout of pumps, but starting with the 300 Boeing began putting one of each on each system, for greater redundency.

What does this mean to you?

Well if we look in our Boeing books you'll notice that A hydraulics power nosewheel steering and B powers brakes (among other things). Being under tow/push you have no use or A system (in fact it can be quite dangerous given the circumstances on the ground to pressurize the A system up during push). You also have a lot of use for B system, namely, the Brakes. This is part of the reason starting with #2 is a good idea. But, there is one more, turning the A pumps off during push is part of many checklists, but having them selected off and actually being off are two different things, you see. EDPs on many Boeing are "powered off", which means if that if you were to lose power during pushback with an engine which powers an EDP running (#1 on 733 and later) you would pressurize that system, with potentially dangerous/deadly results. Losing power with #2 running wouldn't do a thing other than keep B system pressurized when the EMDP goes offline due to the same power (AC) loss.

[quote
The main passengers doors are on the left hand side. On the B707 we always started 3,4,2,1 - you could be winding up 3 and 4 whilst the last pax were boarding, main pax door being closed and the steps were being taken away.
][/quote]

A certain amount of tradition may come into the starting order which was 3,4,2,1. On Lancasters (and the Lincolns I flew) The starboard inner (No 3 engine) was started first because the compressor for charging the air bottle to 450 PSI was installed on that engine. That gave air to the wheel brakes, electro-pneumatic rams for the radiator shutters on the Rolls Royce Merlin engines, slow running cut-out controls, automatic supercharger change and air cleaners. A generator was also installed on both inboard engines.

The port inner had one hydraulic pump that operated the bomb doors, flaps, fuel jettisoning and undercarriage. In addition a compressor on the No. 2 engine operated the autopilot.