Strange noise on 757:

Yesterday I flew SEA to ATL on a Delta 757. I was seated immediately adjacent to the leading edge wing root, right next to the inboard end of the slats.

When the pilot retracted the slats after takeoff, there immediately began a loud whirring sound that seemed to be directly below the floor. This sound was quite intense, and I was debating whether to hit the flight attendant call button, but decided to wait. As I figured, the sound got less noticeable as the aircraft gained speed and the other ambient noises drowned it out, though it was still (slightly) audible.

Naturally, as we began decelerating, the noise seemed to get louder again. Its pitch seemed to change slightly at times, but I could not connect that to anything that the aircraft was doing. I was wondering if the slats would extend normally, but I also predicted that the noise would go away instantly when they did. Of course, that's exactly what did happen.

Normally I would have mentioned it directly to one of the pilots on deplaning, but as on most 757s, we exited thru the #2 door, well aft of the flight deck. I mentioned the episode to a Delta customer service agent inside the terminal, but I suspect that he did not pass the message along to maintenance.

Anyone have any idea what this sort of noise would have been? I've flown plenty of 757s, usually in that general part of the cabin, and never heard that before.

 

Landing gear...?

 

If there was a problem, most likely there would be an EICAS message or STATUS message for the pilots. Unless your an A&P trained in type or a pilot trained in type more than likely it would get blown off. We get all sorts of calls from F/A about "strange noises"

More than likely what you were hearing is the Slat PDU which is in the left wing root.
The flap PDU is in the left wheel well. PDU is power drive unit which gets its signal from the flap lever.

Cheers

 

I'll just mention what I know as a point from which to start. The flaps and slats on all the Boeing jets on which I have worked are driven by hydraulic motors ( if my old head remembers rightly) that rotate a single torque tube continuous from wing tip to wing tip. The torque tube drives retraction and extension shafts by means of a series of gear boxes that make a lot of whirring noises. One can hear the TE flap drive if he sits near the rear spar on outside row seats. The 747 has air driven slats that make a huge howl when they are operating.
Early in the 707 program the main landing gear doors made a loud rumble that was never heard on the prop planes since the LG was out in the nacelles. The company was worried that the new jet passengers would be startled and frightened by the noise and they launched a design effort to put baffles slaved to the MG operation on the forward edge of the wheelwell but after many tests the "Rube Goldberg" machinery was abandoned . There were complaints at times but a simple explanation sufficed. Millions were wasted on that effort.
Switches

 

They also work electrically in the alternate mode in the 757/767. The gearbox you refer to is called the PDU. In the electric mode it takes more than 2 minutes to deploy to flaps 20. I just did a Functional Flight Test out of GYR today in a 757 and they did just that! Great system and backup.

Cheers

 

I can understand a noise while the slats are extending/retracting, but this was after they had finished fully retracting!

I'm very familiar with the noise of Boeing flap motors; in fact on this flight I could clearly hear them during flap operation, even though I was sitting at the other end of the wing root. (Having recalled several crashes on takeoff when the wing was not configured properly, I find that sound very reassuring!)

What, if anything, holds the slats in the retracted position during cruise? That would seem to be the only feasible cause for the noise that I heard.

I should point out that I've been flying in jetliners for 50 years, so I'm quite familiar with the sounds that they should be making!

 

I've experienced the same thing, and as they come in all shapes and sizes, attributed it to Gremlins.
Image Unavailable, Please Login

 

I will talk to the Gremlin Guru in my old group today about gremlinoise. The slats are mechanically held in place after retracting. I suspect some other machinery running.

 

I would guess either a gear door seal or a wing-root fairing seal. Maybe when the slats were retracted it changed the airflow just enough to get a small piece of exposed rubber to start vibrating. They can make quite a lot of noise. Afterwards they usually leave black rub marks that are visible on pre-flight. The quick fix...Metallic speed tape.

Dave

 

I doubt if a gear door was involved, since the noise did not begin until well after gear-up. But your second guess, a wing-root fairing seal, does make a lot of sense. Considering that the noise began precisely when the slats finished retracting and stopped precisely when they were extended again makes that seem like a reasonable cause.

 

Hey, you posted that before I got a chance to! I'm quite sure I thought of it first though...

 

I used to work on these at the delivery center and Renton Flight Line and what I would think you heard was a hydraulic pump in the wheel well there are several in there and at times make a lot of noise. Once that PDU cranks the leading edges in they quit running, there is no continuous movement after that. Sound can easily translate through structure so that the offending component isn't always located at the spot you hear the noise.

I was sitting in a 57 at ATL on my way to SEA one day and heard that familiar sound, of a pump going bad. After you listen to them day after day doing funtional tests on new ones you get to know what they're supposed to sound like, healty or not. In this case not. I told the flight attendant to let the pilot know that a mechanic back here swears he has a bad pump.

I think they knew something was amis already because minutes later maintenance came through and fooled with things and eventually determined we had a bad pump that had been written up previously but they were just milking it. We had to de-plane and get another jet to fly. That's the only time I've ever had to change planes due to maintenance in all the years I've flown.

 

I was once at O'Hare waiting to board an inbound United 757, which was due to arrive rather early. I was puzzled when the plane pulled into the gate about 15 minutes late instead. Although everything seemed normal as the passengers deplaned, the announcement was then made that the airplane was going out of service and that our flight had been cancelled!

We all had to go back to the main terminal and rebook. Fortunately, United did a great job, opening several ticket windows just for our flight, and everyone was rebooked in around 20 minutes or so. In my case, I had to wait about 4 hours for the next flight, but I hadn't been in any particular hurry. (I had a leisurely late lunch in the main terminal, directly under the balcony where Mel Bakersfeld and Tanya Livingston had their offices in the movie "Airport"!)

The story, as I heard it, was that, as the 757 was inbound, one of the flap jackscrews sheared as the flaps were lowered to 15 degrees! The crew flew around for around 20 minutes trying to diagnose the problem, which is why an "early" arrival turned into a late one. Evidently they had to keep the flaps at 15 and land a bit "hot", not a real problem with O'Hare's generous runways. (I suspect that they may have been dumping some fuel during those 20 minutes as well.)

 

You suspect wrong. The 757 does not have dumping capability! unless its a leak! Landing with flaps 15 is Vref30+20 which is not a big deal when compared to a no slat/flapper.

 

Interesting -- I would have thought that all jetliners would be required to be able to dump fuel.

 

there is not a big spread between max gross and landing like the big jumbos. In this case, 255, 500 max gross weight and a max landing of 210,000 lbs. You can land overweight but have to write it up in the log book and do required inspections. If your on fire or need to return ASAP, you dont care if your overweight!

FAR 25.1001 was enacted stating a jettison system was not required if the climb requirements of FAR 25.119 (Landing Climb) and FAR 25.121 (Approach Climb) could be met, assuming a 15-minute flight. In other words, for a go-around with full landing flaps and all engines operating, and at approach flap setting and one engine inoperative, respectively.

Since most twinjet airliners can meet these requirements, most aircraft of this type such as the Boeing 737 (all models), the DC-9/MD80 and Boeing 717, 757 the A320 family and various regional jet ("RJ") aircraft do not have fuel dump systems installed




Cheers