B737-800 Engine Start N2 question
 

My question is moving the engine start lever to IDLE detent preferred once the N2 25% or you wait maximum motoring as I see some captains wait till it reach 29 to 32% and they said this to avoid hot start problems.

also what is the exact definition to the term "Maximum motoring" as I want you to correct my understanding to the above paragraph as I believe it means not more than 25% [[ (if 25% N2 is not possible), at maximum motoring]] this why I wonder why captains wait till the reach higher N2 i.e: 29-32% ?

Well there must be some wisdom in boeing saying that you need to put the start lever to idle at 25% or they would have mentioned to wait till the N2 reached max motoring(which is only if it doesnt reach 25%).
as far as max motoring goes, as it says, it is the stage whn ur N2 has reached an almost constant figure(<1% change in 5 secs)
Even I have encountered some capts who wait for increased motoring(some saying they want to decrease the EGT) i guess its their own personal procedure so better to check with them :}
Cheers!

Sounds like an old version of the FCOM you have there. We used to have the same description a couple years back, but it got changed by Boeing to say max motoring or 20% whichever is higher only. No 25% at all anymore.

my FCOM is the current & its for B737-86N date 27 March, 2009

From the customer code it looks like its been tailored for GECAS (which is a leasing company) or the specific configuration of that plane.

As i said, we had that until two or three years ago but then it got changed by Boeing. So some of your captains might be used to that if they joined after that time. It does indeed decrease thermal stress on the engine and leads to longer engine life (or so Mr. Boeing told us back then).

Not on a 737-800 but as a basic rule, we tried to get our drivers to throw fuel in past minimum start % just as there was a noticable decrease in N2 rotation acceleration. Unless you are high and hot where thin air may make a hot start likely don't do it, you will save the company starters as they take the abuse. We determined that wear and tear in the hot section that some worried about was not a factor.

Not to mention the increased chance of a starter coming apart, if the shaft does not shear and a starter comes apart the damage it can do is extreme, those things spin at insaine RPM's.

It is a bad habit to throw fuel in past max motor.

Reducing EGT with motoring engines was something a BIG fan engine would do. On these smaller versions I doubt it is significant.
Regarding N1%. There is no tailwind limitation on starting engines, but would anyone put fuel BEFORE N1% was showing correct rotation? I note he engines on NG's don't show -ve rotation with a tailwind, only 0%. Then they often beccome +ve aroubnd 27% N2. Thus I wait to confirm all the roatating bits are going in the correct direction before giving them a large shock. Thoughts?

Further, but connected. Inflight Start. I teach to wait until 25%N2 if possible, when at lower altitudes. The 11% minimum means for me it is available at the top of the flight envelope and might well be max motoring up there. Between 5000' & 25000' there might be some line graph which means you'll get only e.g. 17% at a higher FL. But surely, injecting fuel at 11%N2 at low levels can not be a sound thing to do. Could well produce a hotter start, even though the forward speed will help. Thoughts?

RAT 5 is correct the temp reduction in degrees C is in single digits on this engine.

I personally don't move that lever until I see N1 rotation (used to say it in the old checklist). I try and delay start until there is no significant tailwind which means N1 rotation always occurs before 25% N2.

Under normal conditions, one can expect N1 rotation at about 18% to 20% or N2 rotation.

CFM places no tailwind limitation regarding engine starting.

The EEC monitors the engine for impending hot starts while on the ground.

I do remember in a former life, during intermediate stops, we motored the RB211 until EGT was below a certain value.

The variations are endless. I used to fly on 737 with Captains who would wait till max motoring or motor until the EGT was less than x or y or z, despite company published procedures. I think with all of this just do what you are told by your company (and HOPE that your company has cleared the procedure with the manufacturer.)

Regarding tailwind starts, I think I picked up from BM a few years back a simple and effective technique of starting with reversers deployed until N1 rotation - not approved but *** useful in a 60kt tailwind if you cannot be moved.

We would deploy the T/Rs before engine shut down on the JT8, for that exact reason where dust storms and or strong winds were forecast.

While some actions are not approved, sometimes you need to adapt, improvise, and overcome.

Thorough briefing with ground and flight crew is a must. But hey... you do what you gotta do in order to get out of Dodge.

I may have seen that technique with the RB211:E wink wink nudge nudge say no more.

I know a guy who knows a guy who lost a starter on #1 of a dc8. There was a sistership on the ground, got her started by using the thrust of the sisterships #4.

muduckace: firstly I'm trying to decypher your code name and failed. Secondly, this was often a curiosity in the bar as to whether it would work as a get you out of jail technique in some unsavoury dump. But surely, it would give a very hot start blowing large quantaties of hot air down the throat of a healthy colleague. A bit like a few C.P's I've known. Sent we into a red flush rage (inside) a few times.

Was at some time on 747 classics (rolls royce engines) a reccomendataion or requirements to motor the engine until egt had decreased to below 150 and we also had this on the 737 300/400 models for a while. Ex engineers turned pilots would also say that the worst thing to do to an engine is to let it peak and then make adjustments so they might move the start lever to idle at 25 % because the momentum of the start was happening and the acceleration would continue rather than wait for a peak and then add fuel which might begin another acceleration ie engine load. That kind of thinking is similar to the stable thrust requirement of 40% before setting take off thrust. The stability of the engines is more important than the amount of thrust. That is you wouldnt set say 50% then pull them back to 40 and then hit the toga buttons for takeoff as that would be an up down in thrust and engine wear/load which is pointless. What you are seeing in your captains is a reflection and variety of past experiences, past SOPS which are broadly within the requirement to not put fuel in too early, and dont let them burn at the top end of the start, dont run starters for longer than 2 mins. Ask them the question about their actions and they should have a story or 2 for you which will make the sops and individual actions stick in your mind for the understanding you have gained:8

- yes - seen it done a few times. The 'C P's':) output is not so much of a problem if you use the exhaust gas to get the necessary RPM and then cut the 'feeder'. The biggest problem is getting the 'broken one' close enough. Beats runway jump starting:ok:

To add to 'odd' starting techniques, I flew with one Captain who used to move the 737 start lever up so slowly one thought he might have died. Apparently came from the 1-11 and "you get a cooler start if you bring the fuel in slowly". (Don't tell CFM...)

nice try. to bad those new micro-switches (supposedly 6 at the "idle" postion and 6 at "cut-off") only know ON or OFF.:ooh:

Denti, we have the FCOM version from 27 march 2009 for the NG , and for start is says:
" When N1 roatation is seen and N2 is at 25% or (if 25%N2 is not possible), at maximum motoring and a minimum of 20%N2..'
also
'Note: Maximum motoring occurs when N2 acceleration is less tahn 1% in approximately 5 seconds".

Craig F. This ideaq of not 'shocking' the engines has been there with the debate about use of reverse thrust. You fly a long descent at idle, then apply mid-range power for a short while on approach, then pump the engines upto 75% in a burst, then back to idle. What are your thoughts on that, and do you have any factual knowledge about the effect on the core/turbine with the standard technique. I know many airlines are using brakes and idle thrust, but the quick turnround enviroment we now live in needs careful monitoring. I'm amazed that the speedy LoCo's don't have brake temp displays. There is already so much to do on a trun round that checking brake cooling tables amidst all the other parafanalia will most likely be missed. On other a/c this was only necessary with temps above 4, perhaps 5. They would be very helpful if this was so on B737's. However, many Flt Ops chappies on these LoCo's may not have big a/c experience and not choose a customer option apparently to save cost.

I haven't got access to brake cooling scheds right now but I'm pretty sure that on a 30 min turn around in a 737 it's not an issue. Even heavy with 37 degrees celcius ambient . Can someone confirm? Maybe give weights and temps that would cause a problem?

The early 737's with JT8D series engines initially had a start lever actuation at 15 percent N2 during engine start. Complaints were filed to the engine manufacturer that 15% gave high starting EGT that were worrying people. Following further engine testing, the manufacturer recommended a revised figure of 20 percent N2 before the start lever was raised to idle from cut off. The high EGT problem disappeared and everyone was happy.

Then the inevitable happened and some pilots decided they could drop the EGT even further and let the starter wind up to max motoring before introducing the start lever. Of course they hadn't a clue what other technical factors may be affected by the higher N2 rotation.

In other words they disregarded the manufacturer's recommendations in favour of their more expert (in their mind) knowledge. Of course they had not done measured tests to prove their point nor had they written to the engine manufacturer with their research results (apart from personal opinion) to prove the manufacturer recommendations were wrong.

In every airline there are the smarties that insist their personal opinions should be SOP - certainly while they are in command of the aircraft. As a wise man once said "Without data, you've only got opinion".