The start procedure from BEA/BA to ABC changed for a number of reasons. The initial start for 951/953 was as you said 4-3-2-1.. this because these aircraft were pax configured which included the Vanguards rather complex airstairs. Late arrivals of pax have been a bugbear of the airline industry since day 1 so a) you want power to the aircraft systems as soon as possible so 4 was started first.. this being the furthest away from pax entry on the LH side and still allowed access to the freight holds on the RH side. Starting 3 next provided aircon from the compressor fitted to that engine. Meanwhile.. your late pax could arrive and get onboard so the airstairs could be retracted and no.2 could be started.. the other engine providing aircon and finally no.1.
When the aircraft were converted to Merchantmen.. part of the weight-saving measures were to remove the aircon compressor off no.2 and with the majority of cargo being carried internally.. sometimes livestock.. it made sense to start the only engine supplying aircon to be started first.. in this case no.3 so for the Merchantman, the start changed to 3-4-1-2. All 4 engines carry generators but only engine 3 has a compressor. Officially (and I highlight that word) 953c's had a restricted altitude because of the depleted aircon service. Carrying no pax, the same amount of aircon wasn't required but as I said.. some carried livestock hence the alt restriction.
Put simply.. ABC only ever operated Merchantmen so their start procedure was always 3-4-1-2. For BEA.. 951's and 953's remained 4-3-2-1 and the 9 953c's were 3-4-1-2
ATB
DaveB
