pack out of service B737 NG
 

on B 737 NG, when a pack brake down there are two situations:

1 -when it occurs on ground you are limited to FL 250 according to MEL .

2-when it occurs in flight you are not limited according to the QRH.

because when we are on ground ,and after doing the appropriate check list we should consult the MEL.on the other side when we are in flight we do jut the appropriate check list witout consulting the MEL.

Suggest you run a search as this topic comes up in PPRuNe from time to time.

Considerations -

(a) pre-despatch U/S has to be considered from a certification point of view .. hence limitations to preserve a notionally constant risk level with the U/S as an intentional operational matter

(b) in-flight, just one of those things which happen along the way. However, once you have done the QRH things, it is a good idea to check the MEL as that gives you some insight into other considerations which you might not have thought about .. if you are able to comply, there is no reason why you ought not .. and, in so doing, you are reducing those risks which the MEL addresses.

I would go further reference consulting the MEL in flight. You have a problem, e.g. HYD pump or an IRS etc. It is not a 'land at nearest suitable' condition. You continue and fly past your engineering base and ops centre to scheduled destination. Before next departure you consult MEL and find a/c AOG. Oops; perhaps should have gone into engineering base!