Technical Question 757
 

Riding out of Eagle, Co on a 757 the pilot announced and turned the PACS off for take-off. After climbout they were turned back on. Is this for performance? I would think they could run the PACS off the APU and not see a decrease in performance.

Packs off with is for second stage climb performance. Doing it with no APU is because there is a weight penalty for using the APU. (APU door open gives about a 500lb. hit) If the flight was weight critical it probably needed to do it.
Here at DAL, we generally do not leave the APU on, but just take off packs off. We do it on our small engine 767 on every takeoff.

I haven't flown the 757 for three or so years but, in the five years that I did fly it, I don't remember ever taking off unpressurized. Only reason I could see to do that would be if the APU was inop and you had to go bleeds off.

Comfort before efficiency.

Been a few years since I flew the 757 and the 767, I recall a weight penalty for running the APU on the 767 because of the door design, can someone chime in if there is a penalty for the 757 door. Weeding through some cobwebs, but I don't remember there being a takeoff penalty on the 757 for APU on.

Good question from the O.P. !

FF

I have seen it out of high airports like EGE.

We'd do it out of Vegas on a hot day with full airplane in the -300 model. It's a lot heavier than the -200.

.... and even at sea level out of St. Thomas with the down wind take off.

The 2037's have plenty of temp margin. Most of the 2037 s are good for 43,750 lbs with the mods on the engine, but they don't choose to operate it that way. Seems like overkill to flip the packs off, but hey, the jet belongs to someone else. so I'll fly it the way they want it flown. If they've built a better hot and high airplane than the 757-200, I've yet to fly it.

The NTSB's recommendation to pull the engines for inspection for inspection with less than 10,000 hours on the wing probably has something to do with the conservative operation. The CF-34 on a 737 can stay on the jet for 30,000 hours (for comparison).

The early CF6's on the 767 are very tight on temperature margins on take off.

And the MD88's JT8D's just seem to suck under most conditions. It must have been a great engine in the 1950's.

Either Or
 

Packs-Off take-off increases not only available thrust after take-off from hot/high airports, but allows you to take-off at heavier weights. You can also take-off APU-to-Packs, where APU supplies bleed air to air conditioning packs and engines are 'unbled', but 'Packs-Off' allows heaviest weight for t/o. At first clean-up altitude, 1 pack is turned on, then about a minute later, the other. The aircraft is still slightly pressurized on take-off, and packs are back on within 2-3 minutes after the wheels leave the ground.

It used to be a regular event during the summer on the longer flights out of MDW.

and it seems it will be again. saw a Delta 75 take off of 4R from MDW while doing my walkaround saturday. couldn't tell if it was a -200 or -300, though