Okay, here's a question for anyone who has experience with the modern engines on the newer versions of Airbus A320 and the 737 MAX.

I didn't get much chance to work on the 737 MAX before it was grounded (I think I only worked on a Norwegian one maybe a couple of times before they were grounded), but it of course also has fancy LEAP engine of a type broadly similar to that on the Airbus NEOs, which I do work on pretty often. Anyone who has worked on an Airbus NEO when it's starting its engines will know that the engines have a a rather long start up time (allegedly 50 seconds according to the makers but I've never actually timed it), such that crews are always keen to crank them up early in the pushback procedure, to the point that on some flights, I have to firmly tell the crews to 'stand by engines one and two' so they don't start cranking them up whilst the aircraft is still backing off the stand. Even then, when you've allowed the engines to be cranked quite early on the push, you're often stood at the TRP for a while making sure the engines have safely cranked up before you disconnect if you are headsetting one out.

As I understand it, the slow start up cycle is to protect the engine whilst it is still cold in order to prevent wear and tear on the thing, a bit like you don't start revving the hell out of your car's engine two seconds after you've started it. So here's the question: Do the engines take as long to crank up on the 737 MAX, or is there something different about the engines and the lubrication on those fitted to the 737 which means they don't take ages to crank up?

Well I'm clearly not a 737 pilot however there is a excellent video on the max by mentour pilot including start up. You are right, start up takes longer due to the engine ensuring everything is at the correct temperature and in balance before igniting.

Engines from 11 minutes in the vid.

 

The issue is not a cold engine, but rather a hot engine from the previous flight. After shutdown, these engines have issues with its uneven cooling and weight, which apparently leads to something called ‘bowing’ of the shaft. The rotor shaft starts bending after shutdown, which will be bad if the engine is run with a bent shaft.

To mitigate the bending, we are required (airbus) to perform a 3 minute cooldown period after landing. For engine start, an extended dry motoring operation is performed to even out the temps and straighten out the shaft. The length of dry motoring varies depending on how long the engine has been shutdown with that required time displayed to us. There is also a function called ‘dual cooling’ available to shorten the time delay by dry motoring both engines at once prior to starting them individually.

44 minutes ago, KevinAu said:

For engine start, an extended dry motoring operation is performed to even out the temps and straighten out the shaft. The length of dry motoring varies depending on how long the engine has been shutdown with that required time displayed to us. There is also a function called ‘dual cooling’ available to shorten the time delay by dry motoring both engines at once prior to starting them individually.

Aah, that's exactly the insight I was looking for, it explains why the engine seems to be cranking for a long time before appearing to come to life. Thanks for the explanation, makes perfect sense!

This reminds me of Concordes de-bow procedure many decades ago for the very same reason...

Edited February 21, 20206 yr by FDEdev

The 737MAX wil show MOTORING on N2 for about 20%~23% when starting, stick to this RPM range to de-bow the shaft, before up to 25% and fuel in.

The time it takes are variable, normally, if the engine is just shutdown, or it's off overnight, there isn't much uneven heat so time is short,  but if it's shutdown for a while like a typic though flight, it will take much longer.

737 NG starts about 40~60s per engine, MAX might take up to 2~3min per engine.