# How are engines numbered?

On a commercial aircraft, the number of engine is usually 2, 4 or 6. When the technician or LAE was given task to inspect an aircraft engine no 2 on a 4 engine aircraft. Which engine would they go to?

2 different lecturers of mine had 2 different answer for this and both have experience in working with aircraft engine.

Lecturer A said: Lecturer B said:

Which one is right? Requesting help from Technician or LAE. A brief explanation would be really helpful too.

• "Lecturer form commercial industry experience" may be quoting start-up sequence rather than identification sequence (tho' it's still wrong for the 747.) Lancaster start sequence is 3,4,1,2, so start sequence is NOT identification sequence.
– RAC
Apr 6, 2018 at 9:39
• According to my knowledge, aircraft engine would first start on even number on even date and odd number on odd date to reduce wear on just one engine upon starting. For Lancester part that would as well be a new info for me.
– cat
Apr 6, 2018 at 11:13
• @cat Engine start sequence is usually determined by systems. On most aircraft this is the engine that has the hydraulic system that powers primary brakes or parking brakes. I don't know any transport aircraft where it is advised to switch by day. Apr 20, 2018 at 3:05

The Wikipedia article is clear: engines are numbered sequentially, left to right, as seen by the pilot facing forward. That corresponds to the second of your diagrams.

Twin engined aircraft have just engines 1 and 2 on the port and starboard wings respectively.

Three engined aircraft follow the same convention with the number 2 engine in the fuselage centre line.

Since this appears to be an arbitrary convention I'm not sure what more explanation you're expecting.

• Lecturers can be wrong sometimes. Yes, the "lecturer from military aircraft experience" is correct and the other is wrong. You need only to look at flight deck photos of any 4 engine airliner to confirm this. en.wikipedia.org/wiki/Thrust_lever#/media/… Apr 6, 2018 at 6:21
• "...as seen by the pilot facing forward.": If this is important to mention it should also be added "...and not hanging up-side-down (like a bat)"
– Curd
Apr 6, 2018 at 9:35
• Twin-engine aircraft may have them numbered 1+2, OR may label them Left and Right. It's whatever the manufacturer wants. eg. Boeing-Renton uses L+R whereas Boeing-Everett uses 1+2.
– RAC
Apr 6, 2018 at 9:41
• curd, it is actually important to state it from the pilot's view as people would misunderstood it from their point of view which is looking at the aircraft facing them.
– cat
Apr 6, 2018 at 11:10
• @Curd no need to be facetious. It’s a perfectly valid statement as you could easily be “aft looking forward” or “forward looking aft”. There is only one sensible orientation for right way up vs upside down in this scenario. Apr 6, 2018 at 14:37

Aircraft engines are ALWAYS numbered from left to right when viewed from the pilot's seat. Additionally, the start sequence is ALMOST ALWAYS number three, four, two, one. There are several reasons for this sequence: Number three engine starter distance from the battery(s) is the shortest The longer the distance the less amps delivered. Jet aircraft like the B-707, DC-8 had the pneumatic air connection close to the Number 3 engine. Once the number three engine was started it helped pressurize the pressure manifold for assisting the additional engine starts. Remember, a low start pressure could result in a hot start. Secondly, hydraulic pumps were located on the inboards #2 & #3 engines on the DC-4, DC-6, DC-7 & DC-8 as well as the B-707 (the B-747 had hydraulic pumps on all four engines plus an APU but the start sequence remains the same). It's never a good idea to have an engine running without full hydraulic pressure available to the brakes. Thirdly, passengers or cargo is loaded from the left side of the aircraft. By starting #3 & #4 first, last minute changes could more easily be facilitated with #1 & #2 dead. Boarding an aircraft behind an idling engine is always an interesting experience. Starting the #2 next would give additional hydraulic back-up and then allow the fire guard to move to the #1 engine and exit to the left. Lastly, by delaying the start of #1 & #2 engine, interior noise would be kept at a minimum for passengers and crew until all the cabin doors were closed.

747 Thrust Levers, numbered by respective engine numbers.

• Please cite your image sources. Aug 4, 2019 at 1:05