atemp
Topic Author
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Progression of audible alerts

Mon Jun 12, 2017 11:02 pm

Hi all--

I'm researching a scene for a YA novel & screenplay. The scenario involves a large villain-owned business jet e.g. Boeing BBJ powerdiving from 41,000 feet. Problem is, I don't have any flight simulator on hand to try out such a scenario.

The pilot and a muscly goon are brawling with the protagonist on the flight deck. Shots have been fired, and the copilot's out of action. At some point they slam into the instrument panel & disengage the autopilot, then wrestle against the pilot's flight controls and put the plane into a pretty much vertical dive. Oh, and they happen to push the throttles to max.

At the last second (!) the pilot manages to pull up, just skimming the treetops of some Colorado wilderness, but structural damage sustained during the, say, Mach 0.995 powerdive causes something bad to happen, e.g. hydraulics failure on control surfaces, wing to fall off, or whatever.

Far-fetched maybe, but that's fiction for you: coincidences always stack up against the hero, failsafes fail, time is compressed, and everything's cranked up to eleven.

That said, I need to write in some expected and plausible ALERTS, alarm sounds and/or recorded/synthesized voice, to be heard as the plane dives.

What I have (generally in order) so far is:

<AUTOPILOT DISENGAGE ALERT>
"SINK RATE" (?based on barometrics? Repeats several times, then)
"PULL UP"
<OVERSPEED ALERT> and/or "OVERSPEED"
At some point, say within 30 seconds of the start they have maybe only 3000 feet (?) to level off & the ground's approaching way too fast. The pilot manages to throttle back and pull up. At this point, radar's picked up
"TERRAIN TERRAIN" then
"PULL UP"

Again, this is above a wilderness, no controlled airspace or approachable airport.

What I don't have a clue about is when audible alerts kick in, given that the powerdive starts well above 2500'; also, which other alerts might be competing for the pilot's attention as the plane overspeeds, cracks up & ultimately fails.

Thanks for any insight!
 
RetiredWeasel
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Re: Progression of audible alerts

Tue Jun 13, 2017 1:35 am

I'll offer my 2 cents on dive recovery not the warnings. In a vertical dive (-90 degrees) if your speed is .9 mach and you start the recovery at 3000 ft, you're a dead man flying. If you don't pull the wings off you'll hit the ground with rather an abrupt 'splat'. I checked my former F-4 Phantom manual that has dive recovery charts. An F-4 that initiates a 6 G pullup at .9 mach and is in a vertical dive at 11,000 feet would still loose slightly more than 10,000 feet.

So to make your book more realistic, either drastically reduce the speed or back up the start of the recovery altitude quite a bit.
 
N353SK
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Re: Progression of audible alerts

Tue Jun 13, 2017 2:17 am

If you want to be perfectly technical, GPWS alerts such as "sink rate" and "pull up" won't activate until you are less than 2,500 feet above the surface.
 
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Starlionblue
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Re: Progression of audible alerts

Tue Jun 13, 2017 3:50 am

- You don't need to bump into the autopilot disengage bar (it's a bar on the 737 I think). Simply wrestling with the controls forcefully enough will disengage the autopilot. Or there's a button on the yoke.
- As N353SK says, GPWS alerts only happen close to the ground. The GPWS modes are not barometric. They are based on radar altimetry.

In your scenario, you'd get the overspeed warning way before everything else. That one is based on air data. Well, air data that's gone through the air data computer(s). You then wouldn't get SINK RATE and the rest until you were already quite close to the ground.

A more realistic scenario would be to start the recovery around 12000 feet, and then the plane calls out the GPWS alerts once below 2500 feet.

The GPWS modes for reference:
1. Excessive descent rate ("SINK RATE" "PULL UP")
2. Excessive terrain closure rate ("TERRAIN" "PULL UP")
3. Altitude loss after take off or with a high power setting ("DON'T SINK")
4. Unsafe terrain clearance ("TOO LOW – TERRAIN" "TOO LOW – GEAR" "TOO LOW – FLAPS")
5. Excessive deviation below glideslope ("GLIDESLOPE")
6. Excessively steep bank angle ("BANK ANGLE")
7. Windshear protection ("WINDSHEAR")


- Don't forget the autothrottle/autothrust disengagement. Disengaging the autopilot does not disengage autothrottle/autothrust.
- A pedantic nomenclature note: It is "autothrottle" on Boeing and "autothrust" on Airbus. However the levers are thrust levers, not "throttles", on both Airbus and Boeing. We don't control a throttle valve in jets. We control thrust.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Woodreau
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Re: Progression of audible alerts

Tue Jun 13, 2017 3:56 am

Well AF447 took a perfectly working airplane from FL380 into the ocean in less than 4 minutes without having to do a power dive.
Once they were below 15,000ft it was not recoverable.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
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CARST
Posts: 1130
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Re: Progression of audible alerts

Tue Jun 13, 2017 6:27 am

Woodreau wrote:
Well AF447 took a perfectly working airplane from FL380 into the ocean in less than 4 minutes without having to do a power dive.
Once they were below 15,000ft it was not recoverable.


I'm very critical on the pilots of this flight, too, but the airplane was not perfectly working, because the pitot tubes were iced up and the aircraft was showing unreliable speed Information and IIRC not the same speed on both displays. This, together with bad CRM and one pilot coming out of his bunk led to a lot of confusion, the crash and loss of all life on board. But I would not call it a perfectly working airplane, the pitot tubes should have never iced up in first place...
 
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CARST
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Re: Progression of audible alerts

Tue Jun 13, 2017 6:33 am

If you want some real-life exmaples, there have been nose-dive accidents with succesful recovery AND major damage of the airplane, so very much your story. Just read up the story of China Airlines Flight 006.

https://en.wikipedia.org/wiki/China_Airlines_Flight_006

It's only Wikipedia, but it also has links to the official NTSB Report and many News articles of the time of the accident.


More interesting reads...

Recovering from the dive:
http://apstraining.com/recovering-from-the-dive/

and

(Behind a Pay-Wall) A Model for Predicting Aircraft Altitude Loss in a Pull-Up from a Dive:
http://epubs.siam.org/doi/10.1137/1030137
 
BoeingGuy
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Re: Progression of audible alerts

Tue Jun 13, 2017 4:09 pm

N353SK wrote:
If you want to be perfectly technical, GPWS alerts such as "sink rate" and "pull up" won't activate until you are less than 2,500 feet above the surface.


That's not entirely true. The Look-Ahead alerts can activate much higher. What you said is correct for the Mode 1 and Mode 2 radio altimeter based alerts. In reality, in a dive the crew would be getting TAWS alerts higher than 2500 feet.
 
atemp
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Re: Progression of audible alerts

Fri Aug 11, 2017 4:27 pm

RetiredWeasel wrote:
I'll offer my 2 cents on dive recovery not the warnings. In a vertical dive (-90 degrees) if your speed is .9 mach and you start the recovery at 3000 ft, you're a dead man flying. If you don't pull the wings off you'll hit the ground with rather an abrupt 'splat'. I checked my former F-4 Phantom manual that has dive recovery charts. An F-4 that initiates a 6 G pullup at .9 mach and is in a vertical dive at 11,000 feet would still loose slightly more than 10,000 feet. ...either drastically reduce the speed or back up the start of the recovery altitude quite a bit.


Okay, thanks, I can work with that. I could back off to a less-extreme angle than vertical. I read a posting from 2014 claiming that a 737 is "too intrinsically stable" to exceed 60 degrees off vertical, making a 90 degree powerdive aerodynamically impossible no matter what the yoke position. And supposedly the flight-control computers have inbuilt limiters that override a throttle command if airspeed approaches the airframe's rating. That said, the EGT is allowed to exceed the engines' rating at the pilot's command in an emergency situation (better degraded engines than death & destruction).

Sooo... as far as audible alerts go, will an overspeed warning sound if the plane approaches/hits its airframe rating at a given altitude? And will an over-EGT alert be issued? Must I forget about writing in terrain, pull-up etc alerts until the plane drops below 2500'?

For the 737 starting at M0.75, linearly accelerating at 60 deg to M0.82 before the computer throttles back, that's an avg vertical component of 883fps*sin(90-60)=441fps, giving (41000-10000)/441=70 seconds to reach 10000', at which point the pilot must start pulling up. I realize it's more complicated and that mach relative to groundspeed changes with altitude, but do these figures seem in the ballpark?

Thanks for the input so far!
 
atemp
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Posts: 11
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Re: Progression of audible alerts

Fri Aug 11, 2017 5:54 pm

CARST wrote:
If you want some real-life exmaples, there have been nose-dive accidents with succesful recovery AND major damage of the airplane, so very much your story. Just read up the story of China Airlines Flight 006. ...


Okay, so given enough wrestling on the flight deck it'd be plausible to disengage autopilot without hitting a discrete control on the panel or yoke. Check.

The guy at the "recovering from the dive" page (a former USAF F-16 Fighter Pilot & Instructor) calls the engine thrust controls "throttles", but maybe that's jargon solely from the piston-engine aircraft. I'll call out "thrust levers" in the screenplay since that's more visual anyway.

As to the angle of descent, I gather that in an unbalanced-thrust scenario (like Flight 006) it is plausible to roll & pitch the plane into an essentially vertical dive, if only temporarily, until the plane's CG & stability brought it to upright. But pulling 5Gs and chunks of horizontal stabilizer ripping away really did happen, and would be very dramatic in a movie or even novel.

Starting a successful recovery at, say, 12kfeet is consistent with Flight 006's 11kfeet metrics. But viewers would only take about 8-10 seconds of screen time before getting bored, so some dire audible warnings would be necessary in the last 5 seconds to create white-knuckled tension as the pilot struggles to bring her plane's nose up. The recent movie Sully supplied just the right amount of cockpit audibles to add to the tension, but I suppose it's true that those audibles occurred at low(ish) altitude.
 
Woodreau
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Re: Progression of audible alerts

Fri Aug 11, 2017 6:59 pm

Did you want the plane to survive or crash?

There are two more 737 accidents where the planes were flying somewhat normally on approach to landing before their crash - United 585 and USAir 427. Time from incident start to fatal crash were I think 6 seconds and 30 seconds. Although I don't think gpws was installed back then so the cvr has no low altitude warnings before the crash. Just final exclamations and the tape/recording stops.

United 585 https://www.ntsb.gov/investigations/Acc ... AR0101.pdf
USAir 427 https://www.ntsb.gov/investigations/Acc ... AR9901.pdf

But as far as the controls are too stable to permit a 737 from going into a dive. All that is needed is full rudder deflection and full back pressure and full aileron in the opposite direction of the rudder with thrust levers idle. The aircraft will be 90 degrees nose down quicker than if you forced the nose over by pushing the yoke forward.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
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Starlionblue
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Re: Progression of audible alerts

Fri Aug 11, 2017 11:01 pm

atemp wrote:
RetiredWeasel wrote:
I'll offer my 2 cents on dive recovery not the warnings. In a vertical dive (-90 degrees) if your speed is .9 mach and you start the recovery at 3000 ft, you're a dead man flying. If you don't pull the wings off you'll hit the ground with rather an abrupt 'splat'. I checked my former F-4 Phantom manual that has dive recovery charts. An F-4 that initiates a 6 G pullup at .9 mach and is in a vertical dive at 11,000 feet would still loose slightly more than 10,000 feet. ...either drastically reduce the speed or back up the start of the recovery altitude quite a bit.


Okay, thanks, I can work with that. I could back off to a less-extreme angle than vertical. I read a posting from 2014 claiming that a 737 is "too intrinsically stable" to exceed 60 degrees off vertical, making a 90 degree powerdive aerodynamically impossible no matter what the yoke position. And supposedly the flight-control computers have inbuilt limiters that override a throttle command if airspeed approaches the airframe's rating. That said, the EGT is allowed to exceed the engines' rating at the pilot's command in an emergency situation (better degraded engines than death & destruction).

Sooo... as far as audible alerts go, will an overspeed warning sound if the plane approaches/hits its airframe rating at a given altitude? And will an over-EGT alert be issued? Must I forget about writing in terrain, pull-up etc alerts until the plane drops below 2500'?

For the 737 starting at M0.75, linearly accelerating at 60 deg to M0.82 before the computer throttles back, that's an avg vertical component of 883fps*sin(90-60)=441fps, giving (41000-10000)/441=70 seconds to reach 10000', at which point the pilot must start pulling up. I realize it's more complicated and that mach relative to groundspeed changes with altitude, but do these figures seem in the ballpark?

Thanks for the input so far!



You'll certainly get a warning if you overspeed. Note the difference between warning, caution and advisory alerts.

You'll also get a warning if the EGT limit is exceeded. However I'm not sure if FADEC will allow more than momentary EGT exceedance before cutting fuel flow.

Overspeed protection in the thrust control would only be active if the autothrust (autothrottle on Boeing) is active. With manual thrust you get what you set the levers at. However on some aircraft, such as most Airbus, the FBW system has overspeed protection in the form of pitching the nose up.

Regarding EGPWS warnings, you can't realistically put them in above 2500 AGL

Barring structural failures that affect the planform, I suspect it is correct that a modern airliner is too stable aerodynamically to remain in a vertical dive if you simply let go of the controls. In fact there have been several incidents and accidents where the pilots would have been far better off not touching the controls during an upset instead of ineptly trying to fix things, most notably AF447. I'm not saying you should never touch the controls in an upset, just that relying on inherent stability can be a powerful tool. As some instructors are fond of saying, if you find yourself oscillating the plane back and forth with your stick input, let go of the stick and let the plane stabilise before you continue. Same principle as the "oh-s***" method of spin recovery in a 172. Just let go of the controls and it resume stable flight (albeit in a rather steep descent).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
atemp
Topic Author
Posts: 11
Joined: Mon Jun 12, 2017 9:51 pm

Re: Progression of audible alerts

Fri Aug 11, 2017 11:25 pm

Woodreau wrote:
Did you want the plane to survive or crash?


Oh, definitely survive... until some incipient overspeed- or overgee-caused structural issue finally bites, or a wing impacts a sudden rising-terrain obstacle (night flight), or some other horrific failure snatched from the jaws of success.

But all these suggestions are great stuff! The rudder+opposing ailerons+back pressure components causing a forced dive might be hard to pull off all at once with nobody in a control seat during a flight-deck brawl.

Of course if/when my story hits the screen, one hopes that knowledgeable pilots will have been consulted to inform the details of the drama. I wonder if there are 737 addons to the MS Flight Simulator that would let me work out realistic timing of this scenario...
 
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Starlionblue
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Re: Progression of audible alerts

Sat Aug 12, 2017 12:43 am

atemp wrote:
Woodreau wrote:
Did you want the plane to survive or crash?


Oh, definitely survive... until some incipient overspeed- or overgee-caused structural issue finally bites, or a wing impacts a sudden rising-terrain obstacle (night flight), or some other horrific failure snatched from the jaws of success.

But all these suggestions are great stuff! The rudder+opposing ailerons+back pressure components causing a forced dive might be hard to pull off all at once with nobody in a control seat during a flight-deck brawl.

Of course if/when my story hits the screen, one hopes that knowledgeable pilots will have been consulted to inform the details of the drama. I wonder if there are 737 addons to the MS Flight Simulator that would let me work out realistic timing of this scenario...


The PMDG 737 for Flight Sim X is probably as realistic a model as you will get on a PC. http://www.precisionmanuals.com/pages/product/FSX/ngx8900.html

I think the only movie where the flying bits didn't step on my nitpicking nerve was Sully. The way procedures are applied is spot on, with the exception of the master warning not being canceled. Presumably the decision to leave the noise going was for dramatic effect.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Redbellyguppy
Posts: 60
Joined: Sat Apr 01, 2017 3:57 am

Re: Progression of audible alerts

Sat Aug 12, 2017 8:17 am

If you bump the controls on a 737 with ap on you will only put it into AP CWS- control wheel steering- but the ap will still be on. Pressing the disconnect button on the yoke will just get you a loud moaning sheep. The autothrottles shoved forward will ultimately get you an overspeed clacker that sounds like tick-tick-tick-tick rather loudly.
 
Redbellyguppy
Posts: 60
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Re: Progression of audible alerts

Sat Aug 12, 2017 8:18 am

If you bump the controls on a 737 with ap on you will only put it into AP CWS- control wheel steering- but the ap will still be on. Pressing the disconnect button on the yoke will just get you a loud moaning sheep. The autothrottles shoved forward will ultimately get you an overspeed clacker that sounds like tick-tick-tick-tick rather loudly.

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