DrDeke From United States, joined Jun 2005, 827 posts, RR: 1 Posted (2 years 12 months 1 day 5 hours ago) and read 1276 times:
I hope these questions aren't too minor or mundane for this forum, but I figure I'll have a better chance of getting answers here:
Last Thursday I was flying from DTW to ATL on a NW DC-9-50 in either 24-A or 25-A (can't remember which). The front portion of the engine was directly outside my window, and when the engine spooled up for takeoff, a strip of what looked like steam or mist appeared for a few seconds between the engine intake and the outer hull of the aircraft. It appeared that this mist was actually flowing out of the engine toward the hull, although I would think that would be impossible with the engine running.
At any rate, the mist disappeared after a few seconds, but I could not think of any plausible explanations as to what might have caused this. I am guessing that it had something to do with a low-pressure area created in front of the engine intake during spool-up, but I was wondering if this is a common phenomenon to observe and if any of you happen to know what might have caused it.
My other question is this: On both this flight and the return segment, a "whoop, whoop" alarm-style noise was clearly audible over the PA system on two separate occasions during each flight, for about 8-10 "whoops" each.
It seemed to me that this alarm sounded when each engine was being started, although I am not absolutely sure of this. I theorized that perhaps there is a low-speed alarm which sounds before the engines come up to speed. If that is the case, my question is: Why is this audible over the PA?
On the return leg, I was sitting in 2D (dunno why they put me there when I paid for coach, but I'm not gonna complain!) and I could see and hear the FA standing there making the announcement (in addition to the audio from the PA), but the alarm noise seemed only to be audible through the PA, not coming directly from the flight deck or the area in the front in which the FA was standing.
Thanks,
DrDeke
If you don't want it known, don't say it on a phone.
N231YE From , joined Dec 1969, posts, RR: Reply 2, posted (2 years 12 months 1 day 5 hours ago) and read 1249 times:
Quoting DrDeke (Thread starter): I am guessing that it had something to do with a low-pressure area created in front of the engine intake during spool-up, but I was wondering if this is a common phenomenon to observe and if any of you happen to know what might have caused it.
That is correct. May I ask if it was raining, humid, foggy, or had rained previously (wet runways)?
Why to ask so? If the outside air had a high dewpoint (Td) and a temperature that was near or the same as the Td (in other words, the air is near saturated), the high intake velocity created in front of the engine during takeoff will create low pressure, which in turn will cause expansional cooling. The expansional cooling of the air results in the temperature meeting the dew point temperature Td, saturating it, and the formation of condensation. The condensation is the "cloud" you see in front of the engine intake (this is exactly what happens when a cloud forms on top of the wing too).
Quoting DrDeke (Thread starter): My other question is this: On both this flight and the return segment, a "whoop, whoop" alarm-style noise was clearly audible over the PA system on two separate occasions during each flight, for about 8-10 "whoops" each.
I once heard the same thing too. I did not however, make a reference to where it was coming from: I thought it was just some auxiliary fuel pumps that where being used to aid engine start. Someone could probably clarify this.
Fr8Mech From United States, joined Sep 2005, 1556 posts, RR: 1 Reply 3, posted (2 years 12 months 1 day 5 hours ago) and read 1243 times:
Quoting DrDeke (Thread starter): My other question is this: On both this flight and the return segment, a "whoop, whoop" alarm-style noise was clearly audible over the PA system on two separate occasions during each flight, for about 8-10 "whoops" each.
What you're hearing there are the ignitors firing, or more accurately, the interferance in the PA system caused by the high energy impulse to the ignitor. There is a breakdown in the shielding, either the PA wiring (likely) or the ignition system wiring (unlikely).
Someone83 From Norway, joined Sep 2006, 1383 posts, RR: 1 Reply 4, posted (2 years 12 months 1 day 4 hours ago) and read 1238 times:
Quoting Fr8Mech (Reply 3): Quoting DrDeke (Thread starter):
My other question is this: On both this flight and the return segment, a "whoop, whoop" alarm-style noise was clearly audible over the PA system on two separate occasions during each flight, for about 8-10 "whoops" each.
What you're hearing there are the ignitors firing, or more accurately, the interferance in the PA system caused by the high energy impulse to the ignitor. There is a breakdown in the shielding, either the PA wiring (likely) or the ignition system wiring (unlikely).
Heard a similar alarm sound when flying on a SAS MD82 this weekend
DrDeke From United States, joined Jun 2005, 827 posts, RR: 1 Reply 5, posted (2 years 12 months 1 day 4 hours ago) and read 1226 times:
Quoting Fr8Mech (Reply 3): What you're hearing there are the ignitors firing, or more accurately, the interferance in the PA system caused by the high energy impulse to the ignitor.
Oh, that makes sense. I certainly hadn't thought of it. Thanks!
Quoting N231YE (Reply 2): May I ask if it was raining, humid, foggy, or had rained previously (wet runways)?
It hadn't rained that recently, but it may well have been fairly humid; it usually is around here.
Thanks everyone for the info and explanations!
-DrDeke
If you don't want it known, don't say it on a phone.
SlamClick From United States, joined Nov 2003, 9996 posts, RR: 78 Reply 6, posted (2 years 12 months 1 day 3 hours ago) and read 1195 times:
Quoting Fr8Mech (Reply 3): There is a breakdown in the shielding, either the PA wiring (likely) or the ignition system wiring (unlikely).
Don't know as I'd call it a breakdown. It is completely normal in the sense that it has been part of DC-9 lore since airframe #1. It just is audible and no corrective action is needed, nor is there any such action that I'm aware of.
I suspect that it relates to the charge buildup and discharge in the igniter exciters which are large capacitors.
I won't tell you about the sound the forward galley coffee maker produces. Last time I did that my post got deleted for lack of technical merit.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Avioniker From United States, joined Dec 2001, 1098 posts, RR: 16 Reply 8, posted (2 years 12 months 23 hours ago) and read 1143 times:
The loss of "corportate knowledge" over the years is truely amazing.
Below is my post from 25 Nov 05 in response to a similiar question and a reply from "SlamClick". His post illustrates my point and my cause of frustration very well. I teach all day and it's increasingly difficult because the basics have been lost in favor of "fast tracking" and other time and money saving ideas.
I'm old enough to remember when the warning was first incorporated on Douglas (that's pre McDonnell takeover) to warn pilots of an impending hot start due to an engine roll back on startup. The noise was an attention getter to keep the individual with his hands on the fuel lever focused on the task at hand. The old JT8's were nortorious for spooling down after start valve close causing an overtemp in the front of the burner area. It has nothing to do with shielding breaking down or old wiring.
Newer APU bleed takeoffs allowed for higher start rpm before ignition making the warning tone extraneous.
The tone isn't heard on shutdown because it takes the fuel lever raised to make it happen.
Ask some of the older TWA heads (oops never mind, they're all gone).
It's a shame Larry Lamm isn't around to hear all the conjecture. It's "reasoning like that in this thread that caused him to assemble his now famous schematic manuals and the TAFI manual.
Blue on top and listen to the white hair. . .
Quoting Avioniker (Reply 4):
It's the engine below idle warning which is heard in the crew headsets once the oil pressure approaches minimum but the engine is below idle.
It's there to warn the crew of an engine failure after they're running but it was never set up to know the difference between an engine spooling up and one winding down.
Passengers hear the bleed over from the flight interphone amp through the PA.
Hearing it on startup is not a bad thing. Hearing it in flight is not a good thing.
SlamClick's post"
This is really interesting. After initial training, many recurrent trainings, and upgrade to captain, I've never heard of an engine below-idle warning. I'm sitting here reading the most current MD-80 manual in my posession and I can't find any mention of it there. So far I've looked in "Warning Systems" and "Powerplant" and in the "Training" section of the manual regarding engine failure training. No mention of it. Do you have any other reference to it? ATA chapter (powerplant or warning systems) I do have the factory AFM out in my garage, I'd like to look it up.
I suppose it is not mentioned in training because there is no crew action relating to it. Curious why you don't hear it on shutdown and I never heard it with an engine failure. In the first case, I suppose it might be disarmed when the fuel control lever is moved to OFF, but in the case of an engine failure I am at a loss.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
Airgypsy From United States, joined Nov 1999, 128 posts, RR: 3 Reply 9, posted (2 years 12 months 21 hours ago) and read 1140 times:
You can hear this sound replicated in any old style camera flash unit. The recharge ciruit charging up the capacitors. The ignitor units operate in the exact fashion as the flash units. A step up circuit increases the voltage to charge the capacitors through a pulse excitation (DC lacks the pulse of AC) as in the points opening in an automotive iginition system. This excitation reaches RF range close enough to induce a sympathetic current (EMP) into the audio cicuits of the cabin PA system (running cabin length next to the ignition circuit).
When the potential of the capacitors reach the resistance value of the airgap in the excitor, the plug fires. Charging starts again. As the capacitor charges, the voltage value approximates the pitch of the whine required to overcome the voltage stored in the capacitor and boost it to the next higher level, until the airgap...or something very close to that.
Whoop, Whoop, Whoop....
See you next week AVIONIKER
Airgypsy
Lucky42 From , joined Dec 1969, posts, RR: Reply 10, posted (2 years 12 months 20 hours ago) and read 1141 times:
I don't know what avioniker is going off about but what the original post is refering to is the ignitors. Airgypsy is correct. On the DC-9 you can count on hearing it. But avioniler is right about one thing you won't hear it until the fuel lever is raised.
Fr8Mech From United States, joined Sep 2005, 1556 posts, RR: 1 Reply 11, posted (2 years 12 months 19 hours ago) and read 1141 times:
Now I haven't been in the industry near as long as you, Avioniker, but I've never heard of a sub-idle warning system and I've run a lot of classic airplanes, with and without headset on.
I've heard the plugs firing, which incidently fire only with the fuel lever up and the start switch engaged (unless in continous or override or flight mode, etc.).
I remember now that we've had this discussion here before.
I flew ex-TWA DC-9s. I was trained with the TWA syllabus, manuals and checklists. Later I had recurrent & upgrade at Hughes AirWest and Republic. My pilot handbook still said TWA in the header on every single page. Nowhere in the pilot handbook, nowhere in the syllabus and not once in my checkout or in six years flying these machines did anyone ever mention that sound being a warning signal and part of the system. One of these manuals still in my possession has not been revised since 1969. Even it makes no mention of this warning. I guess even by 1969 this was deemed not important.
In short, it is "trivia" a "gee-whiz" factoid.
Now, having had this discussion twice I will probably remember this. If I can get any official confirmation that it is true I may even repeat it, but it is still trivia. It is not operationally important. In short, it is the sort of thing that can be safely dropped from the course outline.
Happiness is not seeing another trite Ste. Maarten photo all week long.
LongHauler From Canada, joined Mar 2004, 1817 posts, RR: 21 Reply 13, posted (2 years 12 months 10 hours ago) and read 1140 times:
Quoting Lucky42 (Reply 10): On the DC-9 you can count on hearing it. But avioniler is right about one thing you won't hear it until the fuel lever is raised.
It was the same on the B737-200. The igniters, although armed, didn't fire until the fuel lever was raised on the engine being started.
It was SOP at the airline I was flying to have the Cabin Interphone live and volume up during start/taxi/takeoff. It was on the Cabin Interphone that you could hear the igniters firing. Funny, I never imagined the passengers could hear it on the PA as well!
On average, more people are killed yearly by falling coconuts (150), than by sharks (10).
Goldenshield From United States, joined Jan 2001, 4011 posts, RR: 13 Reply 14, posted (2 years 12 months 10 hours ago) and read 1140 times:
Going out on a tangent, while not ignitors, you can hear this same effect on the EMB-120 from when the starter is engaged until it switches over to generator mode.
"I heard that there are rumors on the internets" -- G. W. Bush at the second 2004 debate
Lucky42 From , joined Dec 1969, posts, RR: Reply 15, posted (2 years 11 months 4 weeks 1 day 22 hours ago) and read 765 times:
Quoting SlamClick (Reply 12): In short, it is "trivia" a "gee-whiz" factoid.
Yeah Slam I guess I am ignorant also because I have worked on DC-9's for almost 20 yrs and have never heard of what avionker is talking about so it must be something from way past my hiring date...
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