News:

Precision Simulator update 10.180 (14 October 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

Equipment cooling design 767 vs 747

Started by emerydc8, Mon, 26 Feb 2018 06:41

emerydc8

Even though the 767 is only six years older than the 744, it does seem that there was a lot of evolution that went on during that time period.

I have also observed that each airplane has its own gremlins/anomalies. On one of our aircraft, if you pull ground power with the APU powering the aircraft, you get a master warning. On another, the autobrake switch will click off (from RTO) when power is pulled. On yet another, the autobrake switch will click off after reaching 50% N2 on starting the left engine.

There seems to be a complete lack of understanding of what Israeli Aircraft Industries did when they converted the 767s to freighters and modified the equipment cooling system. There's certainly nothing in our manuals about any differences and I am sure that the system is different than the passenger version because I recently talked to one of our mechanics who said that there were valves down there that he had no idea what they did and nothing in our manuals about it. Unlike the 744 where you can at least get a good idea of how the equipment cooling works, the 767 is PFM. There's no venting into the forward lower lobe compartment, but there is some sort of closed-loop system that uses the skin of the aircraft as a heat exchanger. The systems information is just not out there like it is for the 744. Most of our captains on the 767 were previously on the 744, so I'm sure we all have the same misconceptions about the 767 system, given the lack of information on the subject and the fact that the equipment cooling switch has an AUTO, STBY and OVRD -- close enough to the 744 switch but the switches definitely do different things.


Edit HH: Original thread split from here
http://aerowinx.com/board/index.php?topic=4649.msg49588#msg49588

John H Watson

QuoteUnlike the 744 where you can at least get a good idea of how the equipment cooling works, the 767 is PFM. There's no venting into the forward lower lobe compartment, but there is some sort of closed-loop system that uses the skin of the aircraft as a heat exchanger

No doubt the 767 freighter is even more complex than the pax system. I may have a few notes relating to our single 767 freighter, but I can't guarantee it's the same as yours.

Actually, the 744 also uses the aircraft skin to keep the equipment cooling system cold. Whenever the inboard supply valve is open (normal ops), air is sucked from the cavities between the aircraft skin and the cargo sidewall liners. I never quite figured out where the air came from though. There must be a general circulation of air in the sidewalls, otherwise a  vacuum would be generated (For fire/smoke suppression purposes, every join in the cargo plastic sidewall panels had to be taped over, so the air may be coming from elsewhere).

Note also, that the aft crew rest on pax 744s uses a skin heat exchanger on the cabin roof.

Cheers
JHW

emerydc8

To my recollection, there is nothing in my 744 manuals or any other manual that discusses this skin heat exchanger, but I don't doubt you that one exists on the 744. It was my understanding that when the equipment cooling system went into a closed loop, it was just a matter of time before the avionics started to overheat and fail. Maybe this skin heat exchanger would be the answer to controlling the temperature.

For instance, if you were at ETP and you got a main cargo fire warning, the QRH would have you press the cargo fire arm switch, which puts the equipment cooling into closed loop. Granted the checklist would have you land at the nearest airport, but if it's more than two hours away (e.g., LAX-HNL), you are probably going to lose a lot of avionics by the time you get there. At least that's what I believed. But maybe the skin heat exchanger can keep this from happening.

John H Watson

QuoteFor instance, if you were at ETP and you got a main cargo fire warning, the QRH would have you press the cargo fire arm switch, which puts the equipment cooling into closed loop.

Closed, but open to atmosphere. Arming the cargo fire system opens the smoke override valve.

The air from the cold cargo sidewall area goes through a shutoff valve (called the "Inboard supply valve"). This valve is closed when the cargo fire switch is armed or if OVRD is selected. When the smoke override valve is activated, the hot equipment cooling air is simply dumped overboard using cabin differential pressure. In some areas, you would get cabin conditioned air flowing back through the system to atmosphere. On the ground, we are warned about not leaving the system in OVRD mode for too long because there is no differential pressure.


emerydc8

When you arm the main cargo switch, it puts the EE cooling into closed loop. When you arm the lower lobe, it puts it into smoke/override.

John H Watson

#5
QuoteWhen you arm the main cargo switch, it puts the EE cooling into closed loop. When you arm the lower lobe, it puts it into smoke/override.

My mistake... I didn't read "main" (deck) cargo

Would a semi-decompressed aircraft be cooler than a normally pressurised aircraft?

John H Watson

If the 3-way valve on the Freighter is open during a main deck fire, I see that conditioned pack air can be added to the mix, helping cool the equipment.

EE & Main Deck Cargo Fire

emerydc8

I guess we get back to the concept of adding air to a "closed-loop" system. How much conditioned air can you add? Is the system not really closed? When you arm the main deck cargo fire, Pack 1 and 3 are essentially shut down by the SMACCS valve and pack 2 air flow is significantly restricted and sent only to the cockpit to keep the smoke out. I guess some of that air could work its way through the panels and into the closed-loop system, but is it enough to cool the EE or is there a skin-type heat exchanger? It always been a mystery to me.

Looking at your diagram (that's nice by the way), it is clear that conditioned air is able to enter the system. Still, I wonder where the existing air goes. Is it designed to leak and admit new air?

John H Watson

QuoteStill, I wonder where the existing air goes. Is it designed to leak and admit new air?

There are probably hundreds of places where it can leak out, intentionally or otherwise. e.g. I seem to remember a vent in the middle of the overhead CB panel, vent holes in various components (CDU's?) which expelled hot air into empty spaces in the pedestal, etc... A few times I've seen paper stuffed into the gaps between CRT displays because air was blowing into pilots' faces. Some of the equipment cooling plumbing probably has drain holes to let condensation drip away.

I guess if you went looking for this stuff you'd find it, but it's not normally the sort of thing you think about.


Mariano

Don't quote me but, on the 767, if you go to override and later descend for landing (decreasing differential pressure), only equipment on the battery/standby system is "guaranteed" to keep working a bit longer after non-essential equipment has overheated/shutdown.

Will look at manuals tomorrow.

Regards,

Mariano

emerydc8

Thanks. There's nothing in our manuals about it, but I wouldn't doubt what you're saying.

Hardy Heinlin

Good evening,

here's a question re equipment cooling and AC bus 1.

We know that the "EQUIP COOLING" caution message appears on the real 744 when AC bus 1 is lost on the ground. This also happens in PSX.

Can the message be inhibited when AC bus 1 is lost in flight?

If so, I'm not sure which inflight specific condition can inhibit the message in this case.

I have several valve and fan disagreement checks in my message logic. And the equipment cooling selector in OVRD may compensate certain malfunctions. But some components require AC bus 1 to operate.

For example:

The EE cooling supply fan fails when AC bus 1 fails. And if that fan fails, the EE inboard exhaust valve is commanded to close. But it can only close by AC bus 1 power.

That valve can also be commanded to close by setting the equipment cooling selector to OVRD, or by arming the lower cargo fire extinguishing system etc. -- but the valve only runs with AC bus 1 power. So I can't eliminate the disagreement, and the message remains displayed. Which also means: Land at the nearest suitable airport.

When the valve is commanded to close due to a fire or smoke while AC bus 1 is powered, the safety backup will work, and the message will not occur.

In the AC bus 1 failure section the QRH doesn't say "Land at the nearest suitable airport". But it doesn't mention the entire list of AC bus consequences either. So is everything the QRH does not mention necessarily negligible? Each significant consequence of an AC bus 1 failure has its dedicated page in the QRH, and there the QRH may give further hints. Otherwise the same hints would appear several times and bloat this compact book which is supposed to be used for "quick" reference. In other words: The usual sources are too vague to answer the above question.


Regards,

|-|ardy

John H Watson

Wouldn't BAT DISCH MAIN/BAT DISCH APU be enough to land at the nearest airport?

Hardy Heinlin

For these failures the QRH doesn't say "land at the nearest suitable airport", it just says "... battery discharging".
(Although these messages have a ">" caret, they are in the QRH.)

John H Watson

Sorry, I misunderstood the intent of your message...

It's about the oddities of the QRH... not really about the aircraft doing something wrong.

If I disable AC Bus #1  10 minutes after I arm the cargo fire system, I don't get an equipment cooling message in PSX.

Is the AC Bus #1 QRH text supposed to list possible side effects or definite side effects?




Hardy Heinlin

Just AC bus 1 failure in flight without any previous non-normal actions.

The EQUIP COOLING caution will appear and you won't get rid of it.

John H Watson

QuoteThe EQUIP COOLING caution will appear and you won't get rid of it.

Understood.

I thought you wanted to know why the AC BUS #1 QRH procedure didn't list every consequence. Why should it need to do that if the Equip Cooling message doesn't always appear (depending on the scenario). If the Equip Cooling system is not in the correct configuration at a particular time, it will tell you and force you to land.

I earlier thought that the BAT DISCH message would cause you to land, but I guess the Bat Busses are still being powered by the main busses. The QRH or aircraft doesn't care what might happen if AC Bus 3 were to subsequently fail. You don't need to worry the pilot with extra information.

Hardy Heinlin

Quote from: John H Watson on Tue, 21 Aug 2018 12:15
I thought you wanted to know why the AC BUS #1 QRH procedure didn't list every consequence. Why should it need to do that if the Equip Cooling message doesn't always appear (depending on the scenario).

Ah, now I see why you mentioned it :-)

OK, so it does makes sense to trigger the Equip Cooling message when AC bus 1 fails while the cooling system is normally configured. Once the bus is lost the cooling can't be reconfigured anymore (e.g. for a cargo fire), so you should land at the nearest suitable airport indeed.

Does anybody disagree with my conclusion?


Regards,

|-|ardy

John H Watson

Not sure if I agree with your conclusion.

I assumed that the Equip Cooling message appeared because (something like) a fan had stopped working when AC Bus 1 failed. If you then have a cargo fire, the fans may no longer be required, but you need to move the valves to a particular position and they can't move (if they are powered by Bus 1). You still have a problem so the equip cooling message remains.

However,  if you failed Bus 1 after a smoke event, you neither need the fans or the valves to move from their current positions... so you get no equip cooling message.

Can you tell from your code what exactly generated the Equip Cooling message after Bus 1 failed?

Hardy Heinlin

Yes, here's my afore-mentioned example again:

The EE cooling supply fan fails when AC bus 1 fails. And if that fan fails, the EE inboard exhaust valve is commanded to close. But it can only close by AC bus 1 power.

The inboard exhaust valve will disagree with the commanded position, hence the Equip Cooling caution will appear, and related status messages too.