737 FMC "descend now"

[gonnabe]

I recently flew with a captain who criticised me for using the "descend now" function in the FMC (737-800). She argued that it increases wear on the engine as it gets two cool dows - one when you execute descend now and one when it goes to retard.

I have never heard this before. Does anyone else have a take on it?

Cheers

[Johnny Tightlips]

I was always taught to start the descent a bit early at a lower descent rate before using LVL CHG or VNAV for the proper descent. It allows the engines to have a few minutes at a mid thrust setting before going to idle after being in the high 80's low 90's N1 for a few hours. Complete opposite to what your Captain said

[Notso Fantastic]

Our frequent unofficial procedure used to be use Descend Now for a minute or so to prevent one large thermal shock and instead make the cool down a 2 stage process. It sounds like she has an idea of her own. Best thing is remember, and do it her way when you fly with her to stop the bitching about it! It really isn't worth making an issue of- some would say both ways were 'right' and neither way is 'wrong'! In the scale of operating a 737, the difference is irrelevant. She won't thank you for pointing it out, so just say 'yes Ma'am!' and smile!
Long ago, in a place far, far away, on the old 747s, the flight engineer used to chop the throttles violently to make automatic bleed valves operate and it was thought it helped prevent the surge-prone engines surging, but I don't remember the procedure lasted long.

[Callsign Kilo]

Utter tripe! DES NOW, ALT INTV etc are features to be utilised. For descent they allow VNAV to manage a capture of the geometric profile when in PATH or if ATC requires you to DESCEND NOW. Gives you 1000fpm until TOD is reached then, as you say, idle thrust and pitch to maintain path. In the interest of CRM say nothing - it isn't a safety concern. For your knowledge

A) if 'engine cooling' was such a concern then why develop such a function?
B) Not all descents are initially continuous, so thrust and EGTs will vary ie in a step descent, descent at a required V/S or descent with an intermediate off.
C) We live in the age of the modern efficient turbofan. Not Frank Whittle's suck, squeeze, bang, blower!

You hear some tosh on the line. Just nod in agreeance and play to her rules on this one. It isn't worth debating.

[misd-agin]

I thougt "B.S." but realized she might have some unknown knowledge. Contacted a guy that should know(flight test experience) - "B.S., but I'll check with an engine rep."

GE engine rep -

"Yes, I confirm that is BS...there is no detrimental impact to engine health for the CFM or GE family of engines for the descent profile described." (using 'Descend Now' prompt)

[gonnabe]

Thanks for your input guys!
It confirms wat I was thinking myself. I think we are all used to hearing a lot of wierd therories that people have made up themself. I could probably make a long list if I wanted to. Personally I like to stick to what I have read in the manuals but it is also important, in the name of good CRM, to listen to the other person and not to start a big discussion about trivial things.

[Andrén]

Next time she says that just point towards the EGT, even with the engines idle you still have a controlled FIRE inside the engine.

Thermal shocking is a problem with piston engines, not with modern jet engines.

[wiggy]

gonnabe

Personally I like to stick to what I have read in the manuals but it is also important, in the name of good CRM, to listen to the other person and not to start a big discussion about trivial things.

Sometimes best handled by saying something like "really, that's interesting, I hadn't heard/realised that"....and then after the sector going away and double checking the FCOM....

(Actually that reminds me, I must go away and look up something I was told earlier today....)