I realized that some Boeing manuals recommend that the engine number 2 must be started first, is there any special reason for this?

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    $\begingroup$ which boeing planes? $\endgroup$ Commented Aug 12, 2014 at 23:18
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    $\begingroup$ I think it has to do with what side the passengers load from. I think typically the engine on the side without the doors is started first, but don't have a source. $\endgroup$
    – Canuk
    Commented Aug 13, 2014 at 7:42
  • $\begingroup$ There is an interesting discussion here: airliners.net/aviation-forums/tech_ops/read.main/89869 where they cite pilot location, APU and electrical configurations, etc. $\endgroup$
    – Canuk
    Commented Aug 13, 2014 at 7:47

2 Answers 2


The isolation valve in Open or Auto ensures that the Auxiliary Power Unit (APU) bleed air reaches the engine number 2 starter. Engine number 2 start Exhaust Gas Temperatures (EGT) are generally higher.


Engine number 2 is started first because it's on the opposite side of the air bridge. When starting with external air or power it is normal to start Engine number 1 because it is further away from the carts.


According to Flight Crew Operations Manual (FCOM) 3 Standard Operating Procedures (SOP):

Engine 2 is normally started first. It powers the yellow hydraulic system, which pressurizes the parking brake

Looking at the electrical schematic that Boeing provides, we see that Transformer-Rectifier (TR) 3 is powered from the R/H AC Bus. TR 3 is an important TR from which significant avionics are powered. Ergo, you would want to have this TR powered up and running first in case you lose AC power to the aircraft during start.

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    $\begingroup$ Your answer would be of more value to non-experts when you expand it to cover some essential background information and explain the abbreviations. $\endgroup$ Commented Aug 13, 2014 at 11:59
  • $\begingroup$ @ Peter Kämpf - IIRC there is no such rule that states that Engine No 2 has to be started first . Some pilots do start Engine No.1 first . So , I am afraid that's the only background that I am aware of . Re abbreviations , I have amended it . $\endgroup$
    – DSarkar
    Commented Aug 13, 2014 at 13:59
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    $\begingroup$ I will delete this comment if attended to: What is a TR? What is AC? What is an SOP? What is FCOM? etc. I am not an airline pilot. Only an enthusiast. $\endgroup$
    – CGCampbell
    Commented Aug 13, 2014 at 14:54
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    $\begingroup$ TR = Transformer-Rectifier; AC = Alternating Current; SOP = Standard Operating Procedures; FCOM = Flight Crew Operations Manual; The definitions of each one, you may be find at google, some are a bit complicated to explain here. $\endgroup$ Commented Aug 13, 2014 at 15:18
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    $\begingroup$ In airplanes, some systems are projected to be powered with different kinds of currents, take a look in this manual and you will clearly see the difference: smartcockpit.com/aircraft-ressources/B767_Electrical.html $\endgroup$ Commented Aug 14, 2014 at 14:16

Because most people are right handed.

Let me expand a bit on that:

Ancient Boats

Our story begins in ancient times, long before aviation took off.

Early boats were steered with a steering oar attached to the side of the boat. You can see one in the following tapestry:

Tapestry showing ancient boat
(image source: Wikimedia)

Since most people are right handed, the steering oar was attached to the right side of the boat. The Old English word steorbord comes from this steering oar, from which the term starboard for the right side of a ship originates. When docking at a wharf, the steering oar would be in the way. That is why boats always docked from the left side, hence called the port side.

Modern ships use rudders on the centreline of the ship, so they can dock from both sides. The terms port and starboard are however still used today.

Flying Boats

What does all of this have to do with aviation? Enter the flying boat:

Short S-23
(image source: Wikimedia)

In the beginning of aviation, most passenger aircraft were flying boats because very few airports existed at the time, particularly ones with long enough runways for larger and larger aircraft. Flying boats could just land on a river, a large enough lake or the sea. This is also the reason for why we are using many nautical terms in aviation (like e.g. nautical miles for distances) and why pilot uniforms look like Navy uniforms rather than Air Force uniforms (see also What's the reason for pilots wearing uniforms?).

How did these flying boats dock when they arrived at a port? Of course with the port side towards the port. That is why passengers had to board the aircraft from the left side. This tradition lives on until today (see also Why do passenger embark on the left side of an aircraft?).

Before Auxiliary Power Units (APUs) existed, you had start a main engine to get electrical power (without draining the battery) and air conditioning. Since the passengers were boarding on the left side, you had to start a right engine first. Once all passengers were on board and the doors were closed, the left engines were started as well.

Modern Aircraft

Today, most (maybe all) passenger airliners have an APU to power the aircraft (and start the engines). They can also be powered by other means at the gate (see What powers an aircraft prior to the APU being switched on?). This means one could start either engine first today. Yet, it is still common to start the right engine first, e.g. the Boeing 737 FCOM, Normal Procedures, Engine Start Procedure says:

Normal starting sequence is 2, 1.

So why do we still care? In part, due to tradition. But there are some reasons to start the right engine first:

  • APU: The APU uses fuel and this cannot come from the center fuel tank because that could be empty (see Why are fuel tanks in the wings filled first, and why are they used last?). So the designers have to choose either the left or right wing tank for the APU fuel supply. Since it was tradition to start the right engine first, they choose the left wing tank for this. So the APU starts to use some fuel from the left and then when the right engine is started first, it will help to partially balance the fuel.

  • Electrical Power: DSarkar's answer claims that starting the right engine provides power to Transformer-Rectifier 3, which will power the AC systems. This is however not really important since the APU will power all AC systems during engine start anyway. The 737 does not even need any AC power when starting:

    In the GRD position, the engine start switch uses DC power from the battery bus to close the engine bleed air valve and open the start valve to allow pressure to rotate the starter.

  • Hydraulic Pressure: DSarkar's answer further says that starting the right engine is important for providing hydraulic pressure to the parking brake on the Airbus A320. However, all Boeing aircraft can fully pressurize all hydraulic systems electrically (and some pneumatically) before the engines are started.

In the end, it really does not matter any more today. Any Boeing could safely start either engine first. As a matter of fact, sometimes the pushback crew actually asks to start the inboard engine first when turning the aircraft during pushback because they will have to overcome less torque that way.


And there you have it: we start the right engine first because passengers board on the left because boats docked on the left because the steering oar was on the right because most people are right handed.

Bonus: Tail Mounted Engines

The tradition to start right engines first only exists for wing mounted engines. Aircraft with tail mounted engines typically start the engines from left to right. That is why the APU typically uses fuel from the right wing tank for these aircraft (e.g. MD-80, Boeing 717).

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    $\begingroup$ Reminds me of Why do the British drive on the left? (spoiler: it’s all to do with keeping your sword hand free!, so basically for the same reason). $\endgroup$
    – PerlDuck
    Commented Oct 27, 2019 at 11:30
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    $\begingroup$ Sometimes the most complex things have the simplest answers... Is there any documentation of any sort to back this up? I agree 100% (and +1) that it's very logical and makes a ton of sense. Is there something stating that the flying boats would start an engine before all passengers had boarded, thus necessitating not starting the one on the side where they were walking? That would take excellence to greatness! (or is it the other way around?) $\endgroup$
    – FreeMan
    Commented Oct 28, 2019 at 17:55
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    $\begingroup$ @FreeMan This answer to a remotely related question says the order is 3-4-2-1, i.e. starboard first, inner to outer. They say it has technical reasons and is safer and more convenient for the pax because they board from the left. $\endgroup$
    – PerlDuck
    Commented Oct 28, 2019 at 18:25
  • $\begingroup$ @FreeMan I remember that I heard about this in a documentary about flying boats. I just tried to find it again, but couldn't. If I remember correctly, they also didn't start engines during boarding every time, but sometimes it was necessary to get electrical power or air conditioning running. $\endgroup$
    – Bianfable
    Commented Oct 28, 2019 at 18:31
  • $\begingroup$ I think it may be a bit of a stretch to say that most early passenger airplanes were flying boats. $\endgroup$ Commented Oct 28, 2019 at 18:39

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