The Hangar

Lnafziger

21:14

@voretaq7 The answer to your edited question (about airliner cabin pressure) is "It depends on which airplane that you are asking about" now that you removed the 777 tag. Different airplanes have different systems with different levels of control.

voretaq7

21:26

@lnafziger yeah I'm about to edit the answer to reflect that :) (my phone wasn't letting me edit answers)

Lnafziger

@voretaq7 I think that it's a valid question to ask about a system on a specific airplane.

voretaq7

I'd be ok with adding the 777 back to the question as an example too

I'll freely admit I'm being aggressive at genericizing MH370 questions :)

Lnafziger

I'd like to see someone with actual knowledge of 777 systems answer that. For instance, one of the airplanes that I fly would be like: "The pilot can manually open up the cabin outflow valves to dump the cabin pressure but there is an automatic system called a cabin altitude limiter which will close the valves when the cabin altitude reaches 14,700 ft. so that it won't go any higher. The limiter is not electric and can not be controlled by the pilot."

voretaq7

14,700 feet you say... we're the engineers drunk when they picked that, or is that just the strength of the particular spring they put in the outflow valve because "well we have thousands of these...."? :)

Lnafziger

Good question.. I wonder what it is in meters?

roe

21:36

@Lnafziger 14764 is 4500

Lnafziger

Nothing round

Yeah

lol

roe

but I guess the pressure is more relevant

voretaq7

probably - but blowing off the altitude just decreases the pressure differential :)

roe

yeah.. I realized that :)

voretaq7

I could understand if they limited it at 12,500 or 10,000

Lnafziger

21:41

@DeltaLima @flyingfisch For vote-to-delete, at 2k rep you can vote to delete a post if it has been closed for at least two days (otherwise you can flag to delete to inform others that you think that it needs to be deleted). At 4k that restriction is removed if the question has -3 votes and is closed, or the answer has -1 votes.

voretaq7

maybe 14,700 is another "magic hypoxia number" that I just don't have memorized?

oh and @flyingfisch you made me have to read an NTSB report. That's worthy of penance :)

roe

google air pressure 14700 feet and you get

Boeing 307 Stratoliner

{| |} The Boeing Model 307 Stratoliner was the first commercial transport aircraft to enter service with a pressurized cabin. This feature allowed the aircraft to cruise at an altitude of 20,000 ft (6,000 m), well above many weather disturbances. The pressure differential was 2.5 psi (17 kPa), so at 14,700 ft (4,480 m) the cabin altitude was 8,000 ft (2,440 m). The Model 307 had capacity for a crew of five and 33 passengers. The cabin was nearly 12 ft (3.6 m) across. It was the first land-based aircraft to include a flight engineer as a cre...

what the? :)

Lnafziger

Yeah, there was a whole series of events that happened as the cabin altitude went up (I don't fly that airplane anymore). Let me look it up real quick.

Actually, it was 13,700 ft.

voretaq7

13700 is still a funky number :-)

roe

14700 ft is almost exactly 400 mmHg... not that that seems relevant :)

Lnafziger

21:47

@roe Nice! It's probably a differential pressure.

voretaq7

I did find some stuff referencing 14700 feet from an ancient aviation medicine textbook on Google Books - apparently that was the benchmark altitude they used for comparing gas-exchange (sea level / 750 feet & 14,700)

I bet there's probably some long winded "blah blah blank partial pressure of oxygen" explanation for it

Lnafziger

This is for the Learjet 60:

The first indication of a problem is at 8,600 ft +/- 250 ft.:
The automatic Pressurization controller changes to manual mode and the PRESS SYS and FAULT lights illuminate.

At 9,500 ft. +/- 250 ft.:
The emergency pressurization valves are activated, directing bleed air directly into the cabin. (This is automatic and can not be controlled by the pilot)

At 10,100 ft. +/- 250 ft.:
The Cabin Altitude Warning Horn sounds. (And an emergency descent must be initiated.)

At 13,700 ft. +/- 500 ft.:

I used to have all of that memorized, lol.

voretaq7

"directing bleed air directly into the cabin" - I bet the passengers appreciate that but hey at least they're alive to bitch about it?

Lnafziger

@voretaq7 Yeah, when we were empty we would sometimes manually activate the emergency pressurization because it was the only way to get some warm air into the cockpit on the older Lears. It got horribly cold during night flights in the winter for us.

voretaq7

I still maintain there should be a Disney Button - immediately blow the cabin to 13000 feet and keep it there until we land in Orlando. Let the gate crew unload the unconscious monsters.

Lnafziger

21:52

Actually, the 60 (and the Falcon that I fly now) has a cabin dump feature.

voretaq7

I would gladly endure a little hypoxia if it meant the little brat behind me would stop screeching about "I wanna see Mickey" and kicking my seat :-)

Lnafziger

It takes the cabin to 13,700 ft.

lmao

voretaq7

would you have de pressurized if you were flying Bieber and his stoner brigade around? :)

Lnafziger

It's that altitude because the cabin altitude limiters are manual, so they kick in at 13,700 (+/- 500 ft.)

voretaq7

(personally I'd have tossed him out a door and claimed it was a catastrophic in-flight failure...of my freakin patience)

Lnafziger

21:54

@voretaq7 I dunno... At least on the falcon the outflow valves are in the back (unlike the Lear) so it wouldn't be coming forward....

I would absolutely tell them to put it away before I call the police, but then the question is whether or not I actually would. Guess I have to be put in that situation to find out.

voretaq7

oh I would. I'm predisposed not to like the squeaky Canadian bastard, but if he were stinking up a plane I was responsible for? Aw hell no!

Lnafziger

The funny part about that situation is that they must have only brought enough for the flight since they searched them afterwards and didn't find anything. Couldn't they have just done that before they left??

voretaq7

Never carry more than you can smoke or swallow? :)

Lnafziger

I do wonder who told the media about it though. The pilots or the customs agents?

flyingfisch

@voretaq7 oh i see lol

voretaq7

22:00

good question - From the "too much money, too few brains" friends I have I can say it seems like there's a don't-ask-don't-tell mentality among charter pilots

Lnafziger

Yeah, I think they refer to it as "customer service" and they are hoping for tips/repeat business.

voretaq7

There's some business you just kinda don't want. Like potheads, coke fiends, etc.

Piper has apparently found a way to power electrical instruments without connecting them to the aircraft's power bus. They should patent this!

flyingfisch

22:17

@voretaq7 source?

voretaq7

Well my plane has an electric turn coordinator

I've looked through the entire service manual, there's nothing on how it actually gets connected to the power bus

Lnafziger

"magic"

flyingfisch

so you are being facetious?

voretaq7

(which I think means it's wired directly to the bus bar with no fuse/CB)

not at all @flyingfisch - everyone knows aircraft service manuals are always perfect, accurate documents reflecting the state of the aircraft as it shipped from the factory....

(BRB, cleaning up the coffee I just sprayed all over my diagrams)

flyingfisch

@voretaq7 lolk

voretaq7

22:21

I'm writing up the changes for this year's annual (since I couldn't fly today....stupid 30+ knot winds!)

Lnafziger

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voretaq7

22:38

I believe that's one of the ones we can edit on our own so once we have a good list there we can populate the help center page

I was really tempted to make my comment on this question "Hysteria is a terrible way to design aircraft systems."

DeltaLima

23:19

It's amusing to see how many people suddenly know how to solve all issues in aviation. Without having any clue

casey

amusing at first, then tiring

voretaq7

troubling, never amusing

casey

I'm somewhat tired of all of that "why can pilots turn X off" stuff

I just added a comment that basically said "because you trust us, or you wouldn't get on our airplane"

...

voretaq7

because I know how Congress works - they hear this shit and they go "Hey YEAH- why CAN pilots turn off the transponder?! We should legislate that they can't!"

and then one day my transponder shorts out, but it's soldered to the power bus so I die in a fire in the air.

casey

@voretaq7 as long as the CB remains, we can still pull it! I can't count the number of pilots I knew that would pull the parking brake CB to get ACARS to send the "out" time to start the pay clock

since dropping the brake w/o ground coordination could end badly

voretaq7

23:22

@casey they'll mandate that the breaker has to be non-pullable :)

casey

:)

voretaq7

(it bothers me that all my avionics breakers are pop-only...)

casey

I can always pull the avionics bus breaker !

voretaq7

"clearly circuit breakers are a security risk and should not be allowed on aircraft!"

3

@casey and that reminds me I need to crawl behind my panel at some point and see if my avionics switch is a high-current switch, a switch+relay, or a lever-breaker.

casey

:)

voretaq7

23:30

@casey this does remind me of what one of my instructors taught me re: electrical failures though - She asked me what I'd do if we lost the alternator and I started going through the Piper load-shedding procedure & she just reaches over and kills the master. "Isn't this faster?"

casey

in the EMB-145 we didnt have choices there, if we dropped below 3 generators all we had left was what was on the essential busses. no manual load shedding necessary.

voretaq7

Given that you probably have more than 2 radios, a DME and a transponder to worry about, that seems entirely sensible.

....and I hope my "new" DME passes it's bench check on Tuesday. I would miss it if we have to remove it :(

casey

on essential we had only the backup instruments, 1 com, 1 nav, 1 xpdr, (no dme I think but I can't recall). no flaps, no pressurization.

voretaq7

I would assume no DME because running the microwave transceiver is rather power-hungry (about the same as a transponder)

these days you probably get one GPS/NAV/COM and that's your ADF/DME equivalent anyway at like half the power

casey

our unit that normally displayed our NAV/COM freq had a backup display that could draw very basic nav info, and in essential power we'd turn on of the nav/com displays to that, as we lose both PFDs and MFDs

ahh, ADF... we weren't approved to use it for nav or approaches in the jet, but it was good for listening to coast to coast on the few redeyes we flew

:)

voretaq7

23:44

@casey did jets ever start doing the whole gas-discharge display thing that is STILL so popular in GA, or did you guys still have nice mechanical numbers clicking in frequencies?

casey

I always wanted to find a way to call in "Yes, I'm at 37,000 feet and I am about to turn the chemtrail switch on, and I want to tell you all about it"

our nav/com interface was an LCD with a few knobs/buttons

I'll find a pic

voretaq7

The ADF is the single most useless piece of equipment in a cockpit next to a LORAN. (says the guy who did his primary training on a freaking island and had the "here, watching the ADF try to kill us by sending us to Bermuda" demonstration)

casey

voretaq7

The gas-discharge thing bugs me - kx-155s still use it, and the Narcos in my plane use it. Let's crank up the bus voltage to 150+ so we can run little neon lamps in the radio - that seems SMART!

casey

the little displays on either side of the middle are the nav/com/transponder interface/display units

voretaq7

23:48

flat-panel LCD - that seems reasonable

casey

those were, yea. The big displays (the EICAS in the middle, the MFDs on the sizes and the PFDs not show) are CRTs

s/sizes/sides

voretaq7

the CRT thing never made much sense to me either but hey it's a jet, it can afford the weight :-)

casey

The FMS were either LCD or CRT depending on which variant of the 145 you were in

and in the non-XR the backup gauges are steam gauges, this one has the glass backups.

voretaq7

yeah I know the older Boeings have CRTs for some of their displays too (be interesting if you can retrofit them with the flat panels)

ratchet freak

lcd takes much less space than crt

voretaq7

23:51

and weighs less, and sucks less power....

ratchet freak

no reason to not switch then

voretaq7

and the LED backlit ones don't need a HF transformer so there's pretty much NO RF

ratchet freak

unless the video feed was geared towards the crt though

voretaq7

I can give you three: F, A, and A :-) you might not be allowed to do the retrofit without a lot of paperwork :-)

ratchet freak

but you get some space for translation hardware

voretaq7

23:53

especially if it's not a commodity video connection that you can just plug & play - "you wanna modify the flight systems?!"

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