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Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 8:28 am

For example, a 757 built for UPS vs a 757 built for United. In the cabin does the same pressurization exist or is it a little less on the UPS?


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RE: Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 9:23 am

I know that they are usualy maintained at a lower temperature, as there is no need to be heated. but I don't know about pressure.
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RE: Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 9:34 am

On the cargo DC-8's I used to work on, there were two oxygen cylinders, which were used for the crew and whatever jumpseaters were on board.

Most of the passenger 737's I work on now have one cylinder for the crew and the cabin seats have oxygen generators located in the PSU's overhead.
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RE: Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 2:44 pm

If you're talking about cockpit/cabin pressurization, there is no difference in the differential pressure (the difference between cabin pressure and ambient atmospheric pressure) between pax and freighter aircraft.

However freighter crews often seem to adjust the cabin pressurization rate of change to the high side. Meaning the cabin pressurizes and depressurizes quicker. Nominally, when aircraft climb, the cabin pressure decreases 500 feet per minute and during descent, the pressure rises approx. 300 feet per minute. Often on the DC10, the rate knob is turned a bit above normal.

If an aircraft is designed to have heated lower cargo compartments, so will the cargo version. If a compartment cannot be heated due to malfunction and is MEL'd, you cannot transport certain types of freight and will take financial hit.

Fedex carries a lot of race horses along with their handlers and they maintain the main cargo compartment cool for the sake of the horses (or so I overhear the handlers say).

The air conditioning system besides controlling cabin temp and providing ventilation also has the critical role of pressurizing the airplane.
On pax airliners, they operate all the air conditioning packs (unless one was inop of course). But on cargo aircraft, they will use only one or two to save fuel when possible. Now you might think then that cargo aircraft don't pressurize as much as pax planes. But that's not the case, it will pressurize fine but with less ventilation due to less air inflow into the cabin. But don't forget, you've only a handful of people in the aircraft versus potentially hundreds, so as long as satisfactory pressurization is maintained air quality should be OK.

Interestly, the MD11 pneumatic/air conditioning system is fully automatic and runs all three packs when the system is in auto.

Another interesting thing is most of our DC10's are certified to operate only 2 out of 3 packs in flight. A long time ago I researched why and found the answer in the flight manual. Reason being is the smoke detection system on cargo DC10's are certified with only 2 packs on.

Find the fuel totalizer that reads 26600 lbs then go straight up to the cabin pressure panel. Notice the rate limit knob white arrow is aligned with the white "pip" mark. On cargo "10's", it's usually cranked to the increase side for faster pressurization rates.

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RE: Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 3:59 pm

FDXmech you're right, it is better for the horses to have the cabin temp cooler than normal. The amount of heat large numbers give off in a confined area can be pretty substantial and can cause problems, even mortality. When we're shipping horses we have the crew set the temp lower than normal, and still it gets pretty muggy (not to mention smelly!) down on the main deck.

As for the other question, most freighters don't have an airtight door between the cargo deck and the cockpit so pressurization would kind of be neccessary.

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RE: Does A Freighter Have As Much Oxygen As A Pax?

Thu Feb 06, 2003 9:40 pm

Out of curiosity, what does the engine vibration system that's deactivated in the World -10 in that photo do?

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RE: Does A Freighter Have As Much Oxygen As A Pax?

Fri Feb 07, 2003 12:06 am

The engine vibration system (EVM) is an indication system which monitors the vibration of the N1 and N2 rotors by use of accelerometers mounted on the engine. They've been deactivated from every DC10 I've ever seen due to either unreliability or inaccuracy.

But the MD11, A300/A310 and probably most/all newer aircraft have this system activated.

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