DeltaGator From United States of America, joined Sep 2005, 6341 posts, RR: 14 Posted (7 years 10 months 2 weeks 2 hours ago) and read 5628 times:
I flew home to ATL from MLB this afternoon (flight 688) on a Delta MD-88 (reg. N907DE) and I heard a sound I've never heard before. After we backed away from the gate and started to taxi to the runway I heard what sounded like very loud finger tapping. I had headphones on and at first thought I had a nervous passenger behind me tapping their fingers on my seat. It was more of a "thumping" sound kind of like it would sound like if you tapped you fingers on your leather recliner at home.
I was about to turn around and say something to him but when I took my headphones off I could better make out the sound coming from in front of me and down and to the left. I was in row 3 of 1st class on the port side of the plane and there was nobody in the first row in front of me except the stewardesses on the complete other side of the head sitting in their seats by the door.
I was thinking it was something on the taxiway. Perhaps centerline reflectors but it came and went and continued even when off the centerline of the taxiway. It finally went away after we rolled onto the runway.
I'm just curious as to what it was since I have spent many hours in the Maddog and never heard that sound. Any ideas out there folks? I surely hope nobody trapped in the wheelwell but who knows.
"If you can't delight in the misery of others then you don't deserve to be a college football fan."
DeltaGuy From , joined Dec 1969, posts, RR: Reply 3, posted (7 years 10 months 2 weeks 1 hour ago) and read 5606 times:
Just to hazard a guess, but perhaps just some aggressive expansion joints in the taxiway tarmac? I haven't been to MLB in a year, so I'm not sure of the condition of the pavement there, but it could have been something on the jet rattling as a result of it.
MX757 From United States of America, joined Aug 2005, 625 posts, RR: 12 Reply 6, posted (7 years 10 months 1 week 6 days 17 hours ago) and read 5494 times:
It was a damn gremlin in the E&E bay tapping his chin trying figure out what system he is going to f*** with today. Didn't you hear about it? Every MD-80 has one the little bastards running around in them! The gremlin's only goal in life is to make a tech's (like me) life pure hell!
On a more serious note, I thought about what you were hearing and all could think was that I agree with your conclusion. The nose tires were probably hitting the taxiway center lights/reflectors. Either that or one of or both nose tires had a flat spot.
I have a lot of experience on DC-9/MD-80's and I can't think of anything in the a/c's forward section that would make a tapping noise you heard. I've taxied numerous times in several types of a/c and when you run over one of those taxi lights it does make a kind of thud that is audible in the cockpit and the forward section of the a/c.
Nonfirm From United States of America, joined Sep 2005, 434 posts, RR: 0 Reply 7, posted (7 years 10 months 1 week 6 days 16 hours ago) and read 5478 times:
Quoting MX757 (Reply 6): It was a damn gremlin in the E&E bay tapping his chin trying figure out what system he is going to f*** with today. Didn't you hear about it? Every MD-80 has one the little bastards running around in them! The gremlin's only goal in life is to make a tech's (like me) life pure hell!
LongbowPilot From United States of America, joined Jan 2005, 577 posts, RR: 4 Reply 8, posted (7 years 10 months 1 week 6 days 11 hours ago) and read 5425 times:
Gator, it is a phenomina within the flaps of the MD88/90s.
Basically the flaps are fully retracted and there is a slight amount of pressure over what is needed to hold those flaps up. so you hear the relief valve in the flap system popping when the pressure is too great.
The mechanic explained that it is something that takes a short fix in ATL, but nothing to delay or MX the flight.
Okie From United States of America, joined Jul 2003, 2719 posts, RR: 3 Reply 9, posted (7 years 10 months 1 week 6 days 11 hours ago) and read 5425 times:
Quoting DeltaGator (Thread starter): I'm just curious as to what it was since I have spent many hours in the Maddog and never heard that sound. Any ideas out there folks? I surely hope nobody trapped in the wheelwell but who knows.
Been there heard that.
It was explained to me by a jump-riding Mad Dog Captain sitting by me when I questioned him about that noise and seems to make sense.
There is an aux electric hydraulic pump and accumulator in system that pressures the control system when engines (or engine if single engine taxi) are at idle as the slow turning engine/engines do not provide all the quanity of hydraulic volume required. (I also assume that this pump could be used in an emergency)
The sound you hear is the check valve closing when the aux system puts hydraulic fluid in the system and recloses preventing back flow. I have been on some 80's where that pump has been pretty noisy starting and stopping often as soon as it charges the accumulator and system demands are met. The noise stops as soon as the engines spool up for take-off and flight.
While I am not a tail number watcher I have noticed that newer birds are more likely to make the tapping sound while the older birds you seem to hear the whirling of the pump starting and stopping, possibly they have changed their motor/pump design with time.