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I was travelling as a passenger on a 747-8. Around 5 minutes after we left the gate, the captain said we had to return to the gate due to "technical issues with the air-conditioning". The mechanic troubleshot the problem for around 45 minutes, during which the air-conditioning was completely off (but the fans were still running), and it was getting hotter by every minute. Finally, the announcement came and told us the problem could not be repaired on spot. We had to switch plane to continue the journey.

Most airliners have two "packs": left and right. They provide air pressurization and temperature control to the cabin.

  1. Is it feasible to complete an entire flight with one pack inoperative?
  2. If #1 is true, is it legal to takeoff with one pack known to be inoperative? (i.e. does the Minimum Equipment List allow this?)
  3. If one pack fails shortly after takeoff, would the pilot turn around and land, or continue to the destination?
  4. What alert message did the pilots likely got in their cockpit in my case?
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    $\begingroup$ The 747-8 has 3 packs, they are very powerful. $\endgroup$ – mins Aug 24 '16 at 16:41
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    $\begingroup$ The fact that an airliner can be dispatched with some system inoperative (according to MEL) does not necessarily mean that the airline will dispatch it. The airline must consider not just this flight, but the overall disruption to the schedule, so especially if spare plane is available, it is often better to switch and fix it here then risk having to fix it somewhere else where getting a spare plane might be a bigger problem. $\endgroup$ – Jan Hudec Aug 24 '16 at 21:26
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The 747-8 has three packs, and it is feasible to complete the flight with one of them inoperative (in case of aircraft with two packs, cabin altitude can be maintained win any one). The following is taken from the Minimum Equipment list of 747-8:

747-8 MEL

Screengrab from 747-8 MEL

As can be seen, flight is possible with some limitations. In case of a failure in-flight, the flight crew would follow the applicable checklist and decision will be taken whether to continue the flight or not (as altitude is limited). In case of failure, the appropriate warning message will be displayed (the pack, sensor, valve etc.)

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enter image description here

Photo source: own work.

In-flight with the Right Pack inoperative—the button with the sticker.

It is legal; the flight was limited to FL250, and the air conditioning was switched off for the take-off.

If the flight is a long haul, the limited cruise altitude will limit the range. Grounding the plane would make sense then, fuel wise.

The airline and the pilots make sure the MEL (Minimum Equipment List) conforms with the flight's safety.


In-Flight Alert

If a pack fails in-flight, the crew will be alerted. The exact message differs from type to type. The crew then will run the QRH (Quick Reference Handbook) to diagnose and confirm the issue.

They will also call the airline's operations center. If the fuel on board can complete the flight at a reduced altitude, and everything else gets the go ahead, the flight can continue.

For your case, it's hard to know exactly what had failed. It could be a stuck valve, a low pressure alert (leak), an issue with a motor, the logic board for the system, a feeding sensor, etc.

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