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alaska737
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A Few MD-80 Questions

Thu Aug 10, 2006 12:30 pm

Here are a few questions about the MD-80 that i have always wanted to know. I hope I can get a few good answers, thanks in advance

1. Why is the front right door so small compared to other A/C and the other doors on the plane?

2. When looking straight down the fuselage I noticed that the rotating beacon is off to one side, is there a point to this?

3. Every time I fly in a MD-80 shortly after take-off I hear a whirling sound followed by a dull thud near the wing under my feat, no it is not the landing gear and I dont think it is the flaps. it occurs usually about a minute after liftoff and it scares the crap out of me, what could it be?

4. That hole in the base of the tail is the APU right? That would be what makes the whining sound when you board, especially near the tail right?
 
WNCrew
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 12:53 pm

Having worke don the MD-80 it was explained to me that since the circumfrence of the -80 is smaller than that of the 737, the door has to be small so as to not compromise structural integrity since the 1L door is so tall. Otherwise too much of the circumfrence of the cabin would be "door" and not enough solid structure.....but this could be wrong.

Also as to the noice....I always assumed that WAS the landing gear. Sometimes I know I can hear a "whiring" noise on the Boeing and it's the fact that the wheel is still turning a bit....could it be the same thing?
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
WN230
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 12:55 pm

I can give you a guess on points 3 and 4 although I may be incorrect. (I've only flown on a mad dog once hence my lack of knowledge)

point 3 ans: I THINK its the flaps when the FO brings them into the up position. Its really easy to hear especially when you are sitting in row 26 or 27.

point 4 ans: The hole at the base of the tail is indeed the apu exhaust. Depending on where the apu exhaust tube is located in the plane, you can hear it anywhere.

Hope this helps even though I might not know much.  wave 
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aa777sjc
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 1:06 pm

Quoting Alaska737 (Thread starter):
3. Every time I fly in a MD-80 shortly after take-off I hear a whirling sound followed by a dull thud near the wing under my feat, no it is not the landing gear and I dont think it is the flaps. it occurs usually about a minute after liftoff and it scares the crap out of me, what could it be?

I'm pretty sure the whirling sound is the gear being rotated up into position and the dull thud is the gear cover plates (maybe there's a better term?) snapping into position.
 
alaska737
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 1:53 pm

Quoting AA777SJC (Reply 3):
I'm pretty sure the whirling sound is the gear being rotated up into position and the dull thud is the gear cover plates (maybe there's a better term?) snapping into position.

oh right because the MD's cover plates come up slower then say a 737 or something, thats probably it.
 
FBU 4EVER!
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 7:05 pm

I have some 11 1/2 years of flying MD-80's and DC-9's behind me.First: The rotating beacon,and also all antennas along the top of the fuselage are located off to the right because there is a longeron at the apex of the fuselage.Another reason is that the beacon and antennas can be reached and serviced from within the cabin.The 1R door is usually only used for catering supplies.The DC-9 had the larger door size as an option,and some photos of older DC-9's may show this larger door.The fuselage of the MD-80 is almost identical to the DC-9,but the larger door was not,to my knowledge,offered as an option. The thud you hear is almost certainly the Main Landing Gear doors,they are the large inboard doors,and they close rather noisily.As the hydraulic pumps are switched from HI to LO,the main landing gears sag down and rests on these doors. The APU exhaust is the opening just above the right engine nacelle.The intake is located underneath,in front of the ventral stairway.
"Luck and superstition wins all the time"!
 
miamiair
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RE: A Few MD-80 Questions

Thu Aug 10, 2006 8:02 pm

2) AFAIK, none of the Boeings or Douglas airplanes have the anti-collision beacons on the center line on top of the fuselage. This would be BL 0. As FBU 4EVER! has posted there is a longeron/stringer at the center line. This would preclude from any antenna being installed at this location.

4) The hole at the base of the vertical stabilizer is a ram air duct for airconditioning/pressurization system.

The APU is located aft of the pressure bulkhead in the lower fuselage. The intake is below it. There is a ram air door for operation in flight. The exhaust is ducted up the right side of the fueslage to a port above the pylon. A side note to this is that the R/H upper cowling cannot be opened while the APU is operating, because the exhaust will burn the composite cowling.
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AC320tech
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RE: A Few MD-80 Questions

Fri Aug 11, 2006 1:00 am

Quoting Alaska737 (Reply 4):
oh right because the MD's cover plates come up slower then say a 737 or something, thats probably it.

The 737 doesnt have any cover plates or gear doors on the main gear.

For number 4, yes, the hole in the base of the tail is for the APU, and it does make the whining noise when you board the aircraft.
 
miamiair
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RE: A Few MD-80 Questions

Fri Aug 11, 2006 3:42 am

Quoting AC320tech (Reply 7):
For number 4, yes, the hole in the base of the tail is for the APU, and it does make the whining noise when you board the aircraft.

No, the hole at the base of the vertical stabilizer is NOT for the APU. The intake for the APU is on the belly, forward of the ventral stairs.
Molon Labe - Proud member of SMASH
 
N231YE
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RE: A Few MD-80 Questions

Fri Aug 11, 2006 3:47 am

Quoting Alaska737 (Thread starter):
Every time I fly in a MD-80 shortly after take-off I hear a whirling sound followed by a dull thud near the wing under my feat, no it is not the landing gear and I dont think it is the flaps. it occurs usually about a minute after liftoff and it scares the crap out of me, what could it be?

Most likely, its a landing gear door. However, on NW's DC-9's, I notice a similiar sound when the slats are retracted after takoff. As I watch the slats retract, they make a whirling sound, either from hydralics or the airflow of them. Then, as they near their "stow" position, I can hear a thud, possibly a lock for these surfaces.
 
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ThrottleHold
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RE: A Few MD-80 Questions

Fri Aug 11, 2006 3:49 am

Quoting Miamiair (Reply 8):

No, the hole at the base of the vertical stabilizer is NOT for the APU. The intake for the APU is on the belly, forward of the ventral stairs.

May be, but the one at the base of the tail is the OUTLET.

Quoting WN230 (Reply 2):
I THINK its the flaps when the FO brings them into the up position.

Does it make a different noise when the Captain brings them up?? Big grin
 
Dalmd88
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RE: A Few MD-80 Questions

Fri Aug 11, 2006 10:49 am

Quoting ThrottleHold (Reply 10):
May be, but the one at the base of the tail is the OUTLET

So, what are you trying to say? Miamiair is correct. The outlet is on the right side of the fuselage just above the pylon. My take on the noise is the aux hydraulic pump followed by the thud of the gear doors closing.
 
AC320tech
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RE: A Few MD-80 Questions

Sat Aug 12, 2006 12:37 am

Quoting WN230 (Reply 2):
I THINK its the flaps when the FO brings them into the up position

That would have to be a pretty fast climb or the MD-80 procedures are different because I have never heard of pilots retracting the flaps immediately after t/o in a MD-80. Plus, I would be a bit concerned if the flaps made a whirling noise then a thud afterwards.
 
bri2k1
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RE: A Few MD-80 Questions

Sat Aug 12, 2006 2:23 am

Quoting AC320tech (Reply 12):
Plus, I would be a bit concerned if the flaps made a whirling noise then a thud afterwards.

Sometimes I hear a noise that could be described this way when LEDs are retracted. It doesn't add up with the description of when it occured during the climb, though, since (as I'm sure you know) they are the last to be stowed. The whirring is the actuator/motor and the soft thud is when they settle back against the wing.

Quoting Alaska737 (Reply 4):
because the MD's cover plates come up slower then say a 737



Quoting AC320tech (Reply 7):
The 737 doesnt have any cover plates or gear doors on the main gear.

And, I would be surprised if any passnger could hear the nose gear doors. You still hear a whirring and a thud as the mains are retracted, but it's definitely not the doors:


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Silver1SWA
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RE: A Few MD-80 Questions

Sat Aug 12, 2006 8:04 am

Because the original poster mentioned that the noise happens about a minute after takeoff, I will agree with the person that mentioned the possibility of the slats. I have heard something similar as the flaps/slats were retracted, and slats would be more likely to be retracted one minute or so after takeoff than landing gear. Landing hear is usually raised immediately.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
bri2k1
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RE: A Few MD-80 Questions

Sat Aug 12, 2006 11:28 am

It does make sense. To be honest, I didn't re-read the initial post when I got around to replying, but he does say "about a minute after takeoff," which definitely could be the slats. I referred to this post:

Quoting AC320tech (Reply 12):
That would have to be a pretty fast climb or the MD-80 procedures are different because I have never heard of pilots retracting the flaps immediately after t/o in a MD-80.

The OP never said immediately though, he said "about a minute," and, a lot of stuff happens within 60 seconds of takeoff in a jet, and it's very conceivable the flaps could be completely retracted.
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BrowntailWhale
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RE: A Few MD-80 Questions

Sat Aug 12, 2006 1:38 pm

Quoting Alaska737 (Thread starter):
Here are a few questions about the MD-80 that i have always wanted to know. I hope I can get a few good answers, thanks in advance

1. Why is the front right door so small compared to other A/C and the other doors on the plane?

2. When looking straight down the fuselage I noticed that the rotating beacon is off to one side, is there a point to this?

3. Every time I fly in a MD-80 shortly after take-off I hear a whirling sound followed by a dull thud near the wing under my feat, no it is not the landing gear and I dont think it is the flaps. it occurs usually about a minute after liftoff and it scares the crap out of me, what could it be?

4. That hole in the base of the tail is the APU right? That would be what makes the whining sound when you board, especially near the tail right?

1. It's the galley service door and doesn't need to be any larger. I've flown DC-9s with a larger door, so you get a larger one as an option.

2. So it's not in line with the heat exchanger intake.

3. You are hearing the alternate gear pump cycle. It makes a most distinctive whirring sound.

4. The hole is the intake for the heat exchanger.
 
twal1011727
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RE: A Few MD-80 Questions

Tue Aug 15, 2006 7:01 am

Quoting ThrottleHold (Reply 10):
Quoting Miamiair (Reply 8):

No, the hole at the base of the vertical stabilizer is NOT for the APU. The intake for the APU is on the belly, forward of the ventral stairs.

May be, but the one at the base of the tail is the OUTLET.

The "base of the tail" is vague at best.
Going back from the main gear wells the 1st item you come to is a strip water deflector to usher water away from the APU inlet. The APU air inlet is in 3 parts. 2 normal air inlets approx 3 inches wide by 2.5 feet long on either side of the ram air door. The ram air door is an actuated plate that extends into the air stream(inflight start)to aid the starting of the APU - again inflight. Once the APU starts (inflight or on ground) the ram door closes and the APU inlet doors open (they are both sequenced together - ram door open -APU inlet closed. When APU on-speed its vice-versa) The APU exhaust is ported thru the right aft body area to an exhaust port located above the RT engine pylon. (hence APU off when the right engine upper cowlings are opened.)

Behind the APU is the aft stairway door access panel then the tailskid. On the left side of center is the External APU fire control panel. When you get to the tailcone there is an access panel to get to the tailcone evac slide. To the left of that door is the emergency tailcone slide activation handle. Thats it - no other OUTLETS are down low.

The hole at the base of the vertical stabilizer leading edge is the ram air inlet for the pack heat exchangers. This leads directly to the air conditioners (AC PACKS) and it has 2 big 30 HP fans in there. Other than the APU that is the other noise you hear in the back.

Most MD80 operators (if not all) have the Aux hydraulic pump on from pushback to an intermediate altitude (around 10K ft.) It also has a PTU transfer pump/moter to power the left hydraulic system. This is not a true 100 percent power transfer. When the pressure builds up to over 3000 lbs,the relief valve opens and closes. When this does the pressure drops quickly and it sounds like somebody down below hitting the hydraulic plumbing with a hammer. All totally normal

The flaps on the MD80/DC9 are hydraulic powered (unlike Boeings that use jackscrews.) This means that the trailing edge flaps are extremely quiet while intransit. The LED drive plumbing sounds like a toilet filling up and then shutting off with a clunk as the pressure temporarily surges.( sorry - best analogy I could think of.)

The landing gear actuators are quiet too... All the noise is basically airflow noise with 2 clunks as the gear hits the up stops followed by the final 2 clunks as the gear doors close. Remember...the A/C is doing at least 140 knots so air will make alot of different noises as it flows around protruding structures.

Lastly....the fwd galley door (R1) is not a pax entry door so it doesn't need to be regular size. The same goes for the aft galley door (L2) although it is bigger than the R1 door.

The aft stair door can be used as a pax entry door so it is a normal size door.

To sum up.....A/C are not quiet totally smooth operating machines. They make noise. My advise - expect the noises and enjoy them....When they are there - things are working correctly.

KD
 
ilikeyyc
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RE: A Few MD-80 Questions

Tue Aug 15, 2006 8:08 am

Quoting FBU 4EVER! (Reply 5):
The thud you hear is almost certainly the Main Landing Gear doors,they are the large inboard doors,and they close rather noisily.As the hydraulic pumps are switched from HI to LO,the main landing gears sag down and rests on these doors.

So, there are no up-locks on the mains? They rest on the doors?
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AC320tech
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RE: A Few MD-80 Questions

Tue Aug 15, 2006 9:01 am

Quoting Ilikeyyc (Reply 18):
So, there are no up-locks on the mains? They rest on the doors?

If they rested on the doors, the doors would open in flight. The gears are held up by locking mechanisims (AFAIK).
 
474218
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RE: A Few MD-80 Questions

Tue Aug 15, 2006 9:40 am

Quoting AC320tech (Reply 19):
If they rested on the doors, the doors would open in flight. The gears are held up by locking mechanisims (AFAIK).

The Main Landing Gear rests on the door (the door has the up-locks) and the gear floats between up stop and down stop bumpers.

Quoting WNCrew (Reply 1):
the door has to be small so as to not compromise structural integrity since the 1L door is so tall. Otherwise too much of the circumfrence of the cabin would be "door" and not enough solid structure.....but this could be wrong.

The size of the door does not have anything to do with the structural integrity. The door surround structure is what carriers the loads. DC-9's have had cargo doors installed, with no loss of structural integrity.
 
speedracer1407
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RE: A Few MD-80 Questions

Wed Aug 16, 2006 4:28 pm

Just got back from a couple legs as a passenger on MD-80s, and i was surprised at how noisy the slats where on retraction. They made a loud mechanical whirring noise, followed by...well not quite a thud, but close. This happened not long after takeoff; a minute or two seems about right. As on pretty much every flight i've ever been on, the distinctive thud of landing gear retraction happened immediately after takeoff, not a full minute into climb as some have suggested.
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nonfirm
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RE: A Few MD-80 Questions

Wed Aug 16, 2006 4:41 pm

I believe the size of the R1 door on the MD-80 was not and option to have a bigger door.As for the beacon light being off to the side it is to help with the replacement of it as you do this from the cabin thru an access panel without having to remove a whole ceiling panel.The noise you hear is a few things 1 st the bull nose pulleys are in the fwd pit aft section this is where the slat control system is located also it could be the gear doors coming up and the gears resting on the doors.The hole in the base of the vertical is for the air condition system.As stated above as hyd fluid is ported in and out of the wings it does sound like a toilet flushing. airplane 
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FBU 4EVER!
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RE: A Few MD-80 Questions

Sat Aug 19, 2006 6:53 pm

The Main Gear rests on the doors,which has the uplocks,when hydraulic power dissipates after hyd. pump shut-off.The nose-gear has an over-center lock which keeps up all through the flight.It will not sag down on the nosewheel doors.
In conditions of snow- or slush conditions on the runway we use to keep the landing gear extended for a while after liftoff so the contamination can blow off.We also keep the gear extended in case of hot brakes.It might take as much as a minute to get the temp. below limits.
"Luck and superstition wins all the time"!
 
AA737-823
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RE: A Few MD-80 Questions

Sun Aug 27, 2006 2:16 pm

Lots of aircraft let the gear rest on the doors. The 767/757 does, I believe.
 
jeb94
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RE: A Few MD-80 Questions

Sun Aug 27, 2006 4:12 pm

The whirring noise is not the slats. The whirring noise is the hydraulic power transfer unit. Its a hydraulic motor the spins to equalize pressure between the right and left system. It can be used to pressurize either hydraulic system using the opposite system as a power/pressure source. It whirs any time there is a pressure difference between right and left. This is created anytime the unit is selected on and any hydraulic component is actuated. The thud is the landing gear coming up into the wheel wells. You're sitting right forward of them if you're in the wing exit rows. Rows 26 & 27 of most 80s is right about where the main gear is located for most operators. The row location and numbering can vary between operators. The periscope to view the gear is in the floor on the left side of the aisle somewhere under the carpet in this area.

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