Packs Off Takeoff
 

Under what conditions would you do a takeoff with the packs off?
Does this help conserve fuel?

Thrust limited T/O or SOPs require it.

Packs off allows more engine thrust to be used for the take off. The maximum weight you can take off with is determined by the runway length, obstacle clearance and climb gradient which are all affected by the temperature and pressure (aircraft also have max structural take off weights). If a "Packs On" take off gives you a climb obstacle clearance limit of 55t, you will find that with Packs Off you can take off at 56t because you have a little bit extra thrust.

So the conditions when you will use it is when the aircraft weight, the field limits, or the climb limits mean that you need the extra thrust.

SW

We always do packs off take off to save on engine time.

As others have said, Packs OFF provides additional engine thrust, thus the RTOW may be increased.

Yes, it does save fuel, at any time that the fuel burned is providing for OTHER than engine thrust (e.g. Bleed Air, Generators, Hydraulics) the overall fuel use increases. Of course, turning any of the generators or hydraulic pumps off is unthinkable, but the example is there. At low altitude of course, pressurisation is not an issue, but perhaps a comfort issue in hot climates, which, coincidently, is when you'll most likely need to do a Packs Off Takeoff.

Reducing bleed air for pressurisation / air conditioning is a current trend for fuel saving, e.g. the now-common use of recirculation fans to reduce bleed air requirement.

Regards,

Old Smokey

Interstingly RR requires operators of the Trent 700 powered A330 operators to do a periodic packs-on take-off to encourage surge. This is due to a HPC wear issue that can be brought on during a packs on take-off.

So if the operator has an engine go bang during this periodic take-off, they can then apply to AIB to fly-on for about 10 flights with packs-off take-offs before engine replacement.

Brgd's :8

777-300ER has a feature which, when activated via FMC, allows the apu to supply the left pack during TO, and returns to normal engine operation when in flight.

On our airplane we do packs-off takeoffs if APU is deferred and anti-ice is on. If we don't, we'll get a "BLEED MISCONFIG" caution message.

Another approach is to use perf data for PACKS ON (assuming you're not TOGW limited), but select PACKS OFF to reduce EGT/TIT a few degrees. In the long run it's good for engine life.

I'm interested in the RR periodic PACKS ON TO check for stall. Other engines (CF6-50 comes to mind) used to have an "off-idle hangup" (actually a low-energy stall) if packs were off at throttle advance from ground idle. It could be cleared by turning packs on, then off again at half-throttle.