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Sound On The MD-80 During Boarding  
User currently offlineTWAalltheway From United States of America, joined Jul 2001, 144 posts, RR: 0
Posted (13 years 5 months 1 week 3 days 4 hours ago) and read 5599 times:


I recently took a trip on AA from CLE to SGF (Springfield MO) and back....with TWA MD-80s the whole trip. Although each plane was different and differed in age....This part was always the same.

As I boarded and took my seat, I heard the typical sounds of the APU running and the AC running which produces that whine I guess. However, after almost everyone had boarded, there was an abrupt chainsaw type noise that came from below, which was very brief....followed by a new whine starting up which continued until take off. At first I thought it was like the cargo door closing by a motor....but I don't really think so, as this loud noise precedes a high pitched whine that begins immediately after. And all of this happened before the engines were started.

For anyone who has a very good music ear, as you know, the sound of the AC is a "G" pitch. The sound of this new whine was somewhere between a Bflat and a B natural. My God I can't believe I have such a life that I can match any mechanical noise to a musical pitch.

Anyway if anyone has heard the sound I'm talking about, or knows what it is...respond. Thanks.

14 replies: All unread, jump to last
User currently offlineConcorde1518 From United States of America, joined May 2001, 746 posts, RR: 0
Reply 1, posted (13 years 5 months 1 week 3 days 3 hours ago) and read 5576 times:

I never knew it was a G, so i ran over to my alto sax and transposed it and played it... my new favorite note!


User currently offlineLeftypilot79 From United States of America, joined Jun 2002, 455 posts, RR: 6
Reply 2, posted (13 years 5 months 1 week 3 days 3 hours ago) and read 5572 times:

YAY....Im not the only FREAK that hums to find out what pitch it is. Flying truly is an ART.  Smile


User currently offlineJBLUA320 From United States of America, joined May 2002, 3185 posts, RR: 17
Reply 3, posted (13 years 5 months 1 week 3 days 3 hours ago) and read 5569 times:
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Maybe an accidental switch off of the brakes?

I know on the A320, the brakes can be quite alarming if you have never heard them before.


User currently offlineAAR90 From United States of America, joined Jan 2000, 3564 posts, RR: 44
Reply 4, posted (13 years 5 months 1 week 3 days 3 hours ago) and read 5562 times:

You heard the Hydraulic Transfer Pump which is turned on 5-10 minutes prior to engine start/gate departure and remains on until after takeoff (10k or 18k altitude, I don't recall where on the climb checklist). It is turned on again during descent and remains on until after the engines are shutdown at the gate.

*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 5, posted (13 years 5 months 1 week 3 days 1 hour ago) and read 5530 times:

Hydraulic transfer pump? Is that the same as the Power Transfer Unit? But that would not run continuously, it would cycle only when one hydraulic system demand is beyond its capacity and needs power from the other. Or, if only one engine is running.


User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 6, posted (13 years 5 months 1 week 2 days 23 hours ago) and read 5521 times:

From AAL MD-80 MM, ATA Chapter 29-00-00-0 (Description and Operation)

Paragraph C

(1) A backup means of pressurizing all systems is provided by one, electric, motor-driven, auxiliary pump and

>one hydrau-lically operated, power transfer unit.<

The electric motor-driven pump pressurizes the right system only. The power transfer unit transfers power from a pressurized system to a depressurized or sufficiently lower pressurized system.


(3) The power transfer unit mechanically connects the two systems and enables hydraulic power to be transferred from the higher to the lower system (higher pressure side operates as a motor and lower pressure side as a pump) and supplies a flow of hydraulic fluid at approximately 8 gpm and pressure that may fluctuate within 2000 to 3000 psi (13,800 to 20,700 kPa). The unit is controlled by a single motor operating two shutoff valves, one in each system.

>Operation is controlled by a separate switch located on the hydraulic control panel on the First Officer's Instrument Panel.<

The motor operated shutoff valves are also connected electrically to the low level switch on each reservoir, the shutoff valve will automatically close if either system reservoir is below 1-1/3 quarts (1.26 l). The unit and control valves are located in the left main landing gear well.

AAR90 is quite correct in that the switch (S1-236) on the F/O's Instrument Panel is labeled "TRANS" with "ON" and OFF" positions.

The actual nomenclature (To maintenance) of the unit itself is Power Transfer Unit. I don't have an operating manual handy for that fleet to see what it calls it...

At least that's what the maintenance manual and Lamm manual told me. I don't feel like walking across the base to check "The Okie Blueprints".  Big grin

I would say that whatever we (excuse me, I, ??) want to call it, the hydraulic system is the source of what you are hearing. I don't really have a problem with either way of reffering to the unit in question. A DC-10 and MD-11 make the same sort of noise.

"Eagles soar. But weasels never get sucked into jet intakes.."
User currently offlineTWAMD-80 From United States of America, joined Oct 2001, 1006 posts, RR: 3
Reply 7, posted (13 years 5 months 1 week 1 day 21 hours ago) and read 5404 times:

TWAalltheway, I've heard the same sound. Being a horn player, I also figured it to be in the B - B flat neighborhood. If we're thinking of the same sound then it's right before the engines are started up. I think that the sound is from something that initially spins the fan blades then, the second part is the engines spooling up to idle. I don't know too much about the engine operations, but that's my thought. Hey, it's good to know that there are people out there that can hear the pitches of jet engines in their head  Big thumbs up


Two A-4's, left ten o'clock level continue left turn!
User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 8, posted (13 years 5 months 6 days 15 hours ago) and read 5306 times:

The "chainsaw" sound is the PTU being tested by the pilots before engine start-up.It cycles between 2000 (min) and 3100 (max) PSI and the sound you hear is the pump working up the pressure.Immediately after this check the electric Auxiliary Hydraulic Pump is turned on.This maintains a continuous 3000 PSI and is the high-pitched whine you hear after the "chainsaw" sound.
In SAS,we perform the checklist after clean-up if the ATC situation does not dictate otherwise.Engine pumps are placed from HI (3000) to LO (1500) PSI,the two other pumps being placed in OFF position.
The pumps are turned on again at Top of Descent except for the Auxiliary as cold hydraulic fluid after a long flight may create an "overboost" of the system.This pump is turned on during the Approach checklist.

"Luck and superstition wins all the time"!
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 9, posted (13 years 5 months 5 days 20 hours ago) and read 5246 times:

OK, back to the noise before the engines are started.....

What I gleaned from the above is that the Aux pump is started before engines, presumably using ground power or APU. That pressurizes one of the systems. Then the PTU is enabled to pressurize the second system. These would make a lot of noise particularly noticeable when the engines are off. Then the engines start, and once the engine pumps are online, the PTU will stop on its own.

Our company manufactures the Aux Pump and the Engine driven pumps, but not the PTU. The same Aux pump is used on the DC-10, KC-10 and 717.


User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 10, posted (13 years 5 months 5 days 17 hours ago) and read 5239 times:

SAS practice is to test the Aux. Pump which pressurizes the Right Hyd. System,then switch on the PTU to see that it pressurizes the Left Hyd. System,then both are shut down and remain shut down until after engine start.
Retracting the aft stairway requires that the right system is pressurized and we use the Aux. Pump to provide the power.

"Luck and superstition wins all the time"!
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 11, posted (13 years 5 months 5 days 11 hours ago) and read 5228 times:

FBU 4EVER!, thanks -- I'm wondering how the entire start-up sequence goes, vis-a-vis the hydraulics. After the test of the Aux and PTU, they are shut down; then which engine is started first? Are the engine pump EDV's energized when the engines start? Are they then put into 1500 psi mode? When do they switch to 3000 psi? When is the Aux pump restarted? Can you walk me through the process?

Also, I believe some MD80's use pumps (common to the 737) that do not have the 1500 psi option, while others use the old DC-9 pump, that has.

It's sort of strange, I've been designing pumps for over 20 years and never really thought of how they are started up relative to to the engines.


User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 12, posted (13 years 5 months 4 days 15 hours ago) and read 5206 times:

SAS practice is to leave the Engine driven pump switches in HI (3000PSI) after engine shut-down.The right engine is started first,usually,and the pump starts to rotate as soon as the N2 spool rotates,then we start the left.After engine start,the other 2 pumps are switched on.
After clean-up,we place the engine pumps in LO (1500PSI) position and the Aux. and PTU in OFF.They remain in this position until top of descent when HI is selected and the PTU switched on.The Aux. is turned on when reading the Approach V-list.By this time the hyd. fluid will have increased slightly in temperature,and this way we avoid a pressure "bump" in the system.The engine driven pumps need electrical power to keep them in LO.In case of an electrical failure,these pumps will revert to HI pos. automatically.
All our MD-80's have the 3-position pumps (HI-LO-OFF),whereas our MD-90's only have 2-position pumps (HI-OFF).I'm not aware of these pumps having been installed on MD-80's but it might have been an option for late-production planes.

"Luck and superstition wins all the time"!
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 13, posted (13 years 5 months 4 days 8 hours ago) and read 5189 times:

Thanks a lot FBU 4EVER!!

I believe the Delta MD-88's have our newer 737 pumps, which are "inline" piston pumps not the "bent axis" design as on your MD-80's. Those only have a single pressure, 3000 psi, plus EDV (depressurized mode). Your 717's have (unfortunately) our competitor's pump, but is also the "inline" design.

One reason for the dual pressure pumps on the DC-9 (which is also used on many MD-80's) is to increase pump reliability, since those old designs needed to be overhauled at a fairly low number of hours. By keeping them at only 1500 psi for a portion of the flight, they wear less. The newer design, used on the 737 and later, can and do run over 20,000 hours and are only removed "on condition". I believe your pumps are removed every 6-8,000 hours, hard time.

Thanks again,

User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 14, posted (13 years 5 months 3 days 16 hours ago) and read 5154 times:

Finally got an explanation for the difference in MD-80/90 pumps,great!
We pilots very rarely come across this type of info unless we hang around in the hangars asking.

"Luck and superstition wins all the time"!
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