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CFM56 RPM Change At Idle  
User currently offlineDavid_mx From Mexico, joined Nov 1999, 209 posts, RR: 0
Posted (9 years 6 months 2 weeks 4 days 23 hours ago) and read 3056 times:

Hi, I'm sorry if this question was asked before, I made a quick search and found nothing about it...

This question is for 733 MX people around...

Last week I was onboard Azteca 733 flying from MEX to MTY, when they started the No. 2 engine I noticed what I believe was a constant change in RPMs. I can describe it as if you were moving the throttle up and down, but noticed it only when the engine was in idle power. Is this a normal behavior on the engine? Was it a possible failure? Please let me know.

Thank you for your help.

David.

7 replies: All unread, jump to last
 
User currently offlineAvionicMech From United Kingdom, joined Jan 2005, 315 posts, RR: 3
Reply 1, posted (9 years 6 months 2 weeks 4 days 22 hours ago) and read 3024 times:

If it was a windy day this can cause the idle of the engine to fluctuate up and down a bit, but it is usually not very noticeable. How much was the engine speed changing, was it a considerable increase in speed or just a little?

Avionic Mech


User currently offlineDavid_mx From Mexico, joined Nov 1999, 209 posts, RR: 0
Reply 2, posted (9 years 6 months 2 weeks 4 days 22 hours ago) and read 3015 times:

Thanks for your reply Avionic,

The increase was way considerable and the decrease was compared "like if you had the trust reverser for a little bit", because I have been landing or taking off on real windy days and the engine speed change is, as you said, not very noticeable, if not noticeable at all. Also the weather at MEX was great, although I didn't got the METAR that day (I arrived too late, because a 1 hour delay)

Hope this can help, Best regards.

David.


User currently offlineAvionicMech From United Kingdom, joined Jan 2005, 315 posts, RR: 3
Reply 3, posted (9 years 6 months 2 weeks 4 days 22 hours ago) and read 3006 times:

Well it certainly sounds like there could have been a problem there but I am sure it was nothing major as if it was the crew would have pulled back onto stand and got someone to come and look at it. I don't know much about the 733 engines but I am guessing that they are not FADEC controlled like the NG aircraft so they probably have an Idle Reset Solenoid of sorts. What this does is it puts the engine up to high idle under various conditions such as Engine Anti-Ice, Approach flaps etc and if this solenoid was coming to the end of its useful life then it might be flicking between the two positions of high and low idle. But this is only one possibility and I am sure some of the 737 classic engine guys will give you a better clue.

Avionic Mech


User currently offlineDavid_mx From Mexico, joined Nov 1999, 209 posts, RR: 0
Reply 4, posted (9 years 6 months 2 weeks 4 days 21 hours ago) and read 2999 times:

Thanks Avionic,

I have been looking for the technical sites but found nothing, by the way, the maintenance people were looking around (but not sure were, because Gate 9 at MEX doesn't have a good lighting) 10 minutes before our boarding, and we had the security briefing and had to wait like 3 minutes before pushback, weird isn't it? At least It is the first time that happen to me... maybe I'm paranoic... maybe I'm not...  crazy 

Best regards,

David.


User currently offlinePilotpip From United States of America, joined Sep 2003, 3151 posts, RR: 11
Reply 5, posted (9 years 6 months 2 weeks 4 days 18 hours ago) and read 2910 times:

Does anybody think it could have been a situation where the two engines were not "sync'd" up similiar to a multi-twin? Perhaps the two engines were at slightly different power settings causing the reverberations that David heard?

I thought I'd add a little fuel to the fire on this one. I don't know if jets make this sound like props do but if they do?



DMI
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 6, posted (9 years 6 months 2 weeks 4 days 16 hours ago) and read 2877 times:

Go to a340.net then mulitmedia then other cockpit and click on boeing 737 take-off at frankfurt you will hear that sound in question as the pilot advances the throttles a little. I too always wondered about that on the 737 it makes this sound then becomes quiet and then it makes that grrrrr sound as the throttles are advanced.

User currently offlineDavid_mx From Mexico, joined Nov 1999, 209 posts, RR: 0
Reply 7, posted (9 years 6 months 2 weeks 2 days 18 hours ago) and read 2770 times:

Nope, in fact I did see the video and it seems to me that is pretty normal, this sound was way too noticeable... Something related to fuel mixture?

Best regards,
David.


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