737 One Pack INOP limitation

[deserteagle]

Any idea why there is a FL250 limitation if one pack inop before despatch but no such limitation once take off thrust is applied? (till the time MEL applies)

Thanks!

[aviatorhi]

Among other reasons, because you don't want to go out on a flight with a single point of failure keeping you from having to put the entire plane on O2. The odds of 2 packs failing in one flight are rather slim. The other being that, depending on the condition of the aircraft, you may very well have to descend to keep the aircraft cabin below 10K on one pack.

[ghw78]

Longtime since I flew the 737 but on the earlier models the expalnation was something like this.

If a pack failed at altitude, the remaining pack was able to maintain the already pressurised cabin and hence cabin altitude at the appropriate pressure and manage the descent.

If you had only one pack operating when you left the ground, the single pack would be working overtime or even may not have the capability to pressurise the aircraft to the pressure required to mantain cabin altitude at the higher flight levels. The FL 250 limitation was the guaranteed performance level of a pack that was on its last legs.

A single pack in good operating condition may have been able to provide the required capability at levels greater than FL 250 but there was no guarantee that your remain operating pack was a 'good' one or one on its last legs.

[Sciolistes]

I assumed along the same lines as Aviatorhi. It is just risk management.

Redundancy is to protect you in flight, not on the ground. The MEL is applicable on the ground so the restriction goes in there.

The QRH is for in flight and as a singe pack is enough, then there is no need to further complicate the flight and add risk by restricting the aircraft to a lower altitude and burn too much fuel.

[deserteagle]

@ghw78: Makes good sense not to overload the existing pack!

@Sciolistes: Also agree on the point of fuel consumption as I had always thought. Planning for a routine flight and trip fuel and coming down to FL250 you are going to burn at least 8 to 14% percent more like the book says. Would put in a really unwanted situation!

[Checkboard]

It's been answered before:

QRH:
Chance of a second pack failing during the remainder of the current flight vs. the (minor) problem that would cause.

MEL:
Chance of a second pack failing during an entire flight, or series of flights (10 day MEL limit), said chance can be reduced by a FL250 limit - and unlike the QRH situation that can be fuel-planned before take-off.

Resolution - if the pack fails, and you have the spare fuel, it could be prudent to descend and reduce the load on the remaining pack. Doing that may save you descending (with a slow depressurisation) should the second pack drop off line, but it isn't required (and as it isn't required, then a diversion if you don't have the fuel isn't required.)