I've heard references to pilots (possibly accidentally) doing this. I'd imagine that a Trent-powered 77 APU (which I believe can start both simultaneously) would struggle starting both engines simultaneously off the APU with the PACS on. Same with the 74 starting 2 engines at a time.

I seemed to recall that a pilot on this forum (correct me if I'm wrong) cranked an engine with the PACS on. From my recollection, he said it spooled for much longer, but eventually lit off at a much later time after it was spooling 3 times the normal time.

Are there aircraft with APUs that are incapable of lighting off an engine (or engines) with the PACS running? I'm guessing this isn't SOP for anyone's airline.

Just curious. Anyone ever done it in a reasonable situation (ie, the aircraft has been delayed, the cabin temperature is significantly uncomfortable, and it would be un-desirable to turn off the PACS powered by APU, with already sweating passengers. Does this/can this happen without breaching SOP? What types could be capable of this?

Can't recall at this moment but I believe we can start the 787 with the CACs running?

BravoOne wrote:

Can't recall at this moment but I believe we can start the 787 with the CACs running?

My error as the CACs are commanded OFF during the start and remain off for 2 minutes after the start.

777 and 787 automatically turn the Packs off during Engine Start, IIRC. Other Boeing models you have to manually turn them off.

I assume you meant on the ground, but you can do a windmill start of any engine in air with the Packs on.

I THINK on the 747, you can turn off 2 of the 3 pacs and start a single engine without issue. (if the APU can provide enough air for a dual-engine start it should manage 1 engine and a pack.

Though, you may as well just get all motors running quicker, and get all 3 pacs on, would probably be less noticable vs the longer startup times

Long time ago, as an F/O in training, I forgot to switch off the Packs before start in an Embraer 145, which resulted in a 'hung start' and eventually start abort.

Once the error was discovered and corrected, the subsequent start was ok.

Normal procedures at my former airline for the both the 747-400 and 200 was to turn off the packs just prior to engine start.

However there was a technique used occasionally in the 200 where you could start 2 engines on one side, then close the pneumatic isolation valve on that side, then turn the one pack on that side on, so the engines were providing the air for that pack. The cabin would get a little relief while you then started the other side engines using the APU air. Edit: I should add-- in the 200 you started one engine at a time.

Never tried it but I suspect you could do this on the B727.


We would powerback and taxi out on two engines #1 and # 2 to save fuel, prior to starting #3 if you closed the left APU / Center engine bleed you could
leave the left pack running, it would pull air from the left engine bleed, and the APU could supply air for engine start through the open right APU / Center engine bleed.


As I said, I never tried this but don't see why it wouldn't work. a bit similar to your scenario RW.

In the good old days on the Tristar there was a flight engineer sitting there who controlled it all himself. In the Middle East, packs were required. So the standard engine start was as follows.
The APU was used and it was supplying Nbr 2 pack. Nbr 3 engine start selected. The engine winds up and the F/E monitors the N3. If it goes above 20pc then the pilots start the engine with the pack on, if it doesn,t then he selects the pack off until the engine start valve closes, then pack back on and nbr 3 isolation valve closed and select start on nbr 2. When nbr 3 is stable, select nbr 3 pack on. Same procedure on nbr 2 start. Select the pack off if requitred, and on when not.
There were two reasons. First keep the packs on as much as possible. Second, the APU always produced air. It was much better to use it than allow the APU bleed to close. When the APU bleed closed, a surge valve opened and bled the air overboard.
We always tried to avoid this happening, There was a big whoosh of air outside, and the APU didn,t sound very nice!!
A Flight Engineer could control all this easily. The RB211-22B was difficult to start at times as the surge margin was very small, but turning the pack on or off could sometimes produce a successful start!, especially when it was hot outside.

On the 747-400 it is normal procedure to start two engines at once (we do 4 and 1 followed by 2 and 3) while keeping one pack on. The only caveat is that we have to keep an eye on duct pressure as at higher altitudes as depending on the airplane the APU is unable to handle the load. The -8 however requires us to turn off all packs for engine start. Also, if anyone cares, neither airplane automatically turns off the packs during engine start.

Packs-off for a normal start, on the 767 too.

Tristarsteve wrote:

In the good old days on the Tristar there was a flight engineer sitting there who controlled it all himself. In the Middle East, packs were required. So the standard engine start was as follows.

Fascinating stuff. There's a special place in my heart for the L15.. Growing up, I always found myself on one from ATL to LGW throughout my childhood to see my family in London. One of L15 captains became a family friend. Upon his retirement flight, I planned his party at ATL. Wouldn't you know it? I ended up getting to jump-seat under the water cannons! What an amazing machine.. L10 Operation procedures like what you shared in the Middle East, always surprise but don't shock. The aircraft has Kelly's fingerprints all over it. Thank you for sharing.

RetiredWeasel wrote:

However there was a technique used occasionally in the 200 where you could start 2 engines on one side, then close the pneumatic isolation valve on that side, then turn the one pack on that side on, so the engines were providing the air for that pack. The cabin would get a little relief while you then started the other side engines using the APU air.

I'm sure many-a-passenger appreciated that technique when you implemented it. My only experience with a 747 was as a passenger on a 747-100 I was chartering to a bunch of high school students for a senior trip to Cancun. Why they allowed two college students to manage such a task is retrospectively infathomable today, and to this day my buddy and I are still in disbelief that the Tour company let us do that. I remember being on a hot tarmac in Cancun when the packs were switched off. Seemed like an eternity for all 4 engines to spool up to get the packs online.

Max Q wrote:

We would powerback and taxi out on two engines #1 and # 2 to save fuel, prior to starting #3 if you closed the left APU / Center engine bleed you could
leave the left pack running, it would pull air from the left engine bleed, and the APU could supply air for engine start through the open right APU / Center engine bleed.

Wow, you would Powerback with 2 engines running on the 727? I always love your 72 stories, Max. Were you only in the front two seats during that time? My understanding is that the other 727 advocate here, tb727 was in the third seat at some point. I've always wondered what it would have been like to have your nose pointed starboard to an instrument panel, with no windows, every time you go to work.

Off-topic, but for those here that flew 3-crew cockpits, what (if anything) do you feel has been lost to automating out the F/E?

Best wishes, and thank you for sharing!

That's easy. Another set of eyes and a brain to absorb what's happening at anyone moment. Keep in mind the the FE seat rotates forward so you don't have to look at the panel ll the time:)

And keep in mind that sitting sideways staring at the panel was the entry level position at several airlines from '85 on. As BravoOne mentioned, for takeoff, the FE would swing his seat around and face forward for takeoff and landing. At my former airline, he would also fine tune the power levers on takeoff to match the T/O epr's. Sitting sideways on the 742 was actually quite comfortable as you could slide your chair way back and prop your feet up on your desk and try not to go to sleep.

Only professional engineers (PFE's) really lost their jobs due to two man cockpits and even a lot of those were hired on as pilots with their same seniority if they got their tickets.

RJS, as R Weasel described well, starting as an FE was the normal entry level position for most airlines in the US, I was an FE or Second Officer for four years before upgrading to the First Officer position on the same aircraft, the lovely 727.


Yes, we would powerback on two engines all the time, there was a weight limit for this, above a certain weight you's have to use all three for the powerback but you could then shut down #3 for taxi if there was a long wait and you wanted to save fuel (#3 was shut down as it did not have an engine driven hydraulic pump)


You could taxi on one engine if you were light enough but powerbacks required at least two otherwise you'd be using so much thrust it would be a FOD hazard and to ramp personnel.



Where the FE was really invaluable was during an abnormal or emergency situation, he or she significantly lowered the workload, at my airline when these happened the FO's job was solely to fly the aircraft and communicate with ATC while the Captain and FE worked their way through the problem, it was a good system.



Automation has helped but it can get a bit busy sometimes, especially with an aircraft problem, fortunately on most long haul routes there's an extra relief pilot
on board to help out and it's standard practice for them to be in the cockpit for take off and landing just 'in case'

One cannot help but think that the Swissair MD11 accident would have benefited from a 3rd pilot to assist in what was an extremely difficult situation. Just because its legal does not mean its smart.

All of the Airbus FBW automatically schedule the packs off and on during normal engine start.

About the only time you are going to get a hot cabin is in the event of an inop APU and external air is not available or unable to keep up. There are a few strategies that can be used,including running the packs from a ground source. With the APU inop you would start half of your engines at the bay and pushback, and start the others normally. Between starting the first engine, and by the time you complete a pushback there is normally enough time to have the packs running off one engine to supply some cool air before doing a crossbleed start which takes no time.

Most aircraft are able to have the packs running inflight for an engine start, the airflow through the engine provides assistance to start.

BravoOne wrote:

Can't recall at this moment but I believe we can start the 787 with the CACs running?

The B787 packs are electric and do not require Bleed Air to operate

Hi didn't say they did, just indicated that one cannot run the packs and start the engines at the same time.

BravoOne wrote:

One cannot help but think that the Swissair MD11 accident would have benefited from a 3rd pilot to assist in what was an extremely difficult situation. Just because its legal does not mean its smart.

Wouldn't have done a thing. There was nothing the crew could have really done about the fire, and according to the Canadian TSB:

From any point along the Swissair Flight 111 flight path after the initial odour in the cockpit, the time required to complete an approach and landing to the Halifax International Airport would have exceeded the time available before the fire-related conditions in the aircraft cockpit would have precluded a safe landing.

Now, I'd argue that perhaps an immediate emergency descent would have been a better choice of action, even if in hindsight they couldn't have made it.

GTAEo7 thanks for your thoughts but it's a safe bet that I have forgotten more about this accident than you will ever know.

BravoOne wrote:

One cannot help but think that the Swissair MD11 accident would have benefited from a 3rd pilot to assist in what was an extremely difficult situation. Just because its legal does not mean its smart.

Always wondered why they couldn't make it, why dump fuel, fire and smoke, put her down now. If they would have realized how bad it would get, thought they had time to perform a emergency descent.

zeke wrote:

Hi didn't say they did, just indicated that one cannot run the packs and start the engines at the same time.

Is that due to the packs putting a load on the electrical system, sufficiently high to not crank the motor concurrently?

rjsampson wrote:

I've heard references to pilots (possibly accidentally) doing this. I'd imagine that a Trent-powered 77 APU (which I believe can start both simultaneously) would struggle starting both engines simultaneously off the APU with the PACS on. Same with the 74 starting 2 engines at a time.

I seemed to recall that a pilot on this forum (correct me if I'm wrong) cranked an engine with the PACS on. From my recollection, he said it spooled for much longer, but eventually lit off at a much later time after it was spooling 3 times the normal time.

Are there aircraft with APUs that are incapable of lighting off an engine (or engines) with the PACS running? I'm guessing this isn't SOP for anyone's airline.

Just curious. Anyone ever done it in a reasonable situation (ie, the aircraft has been delayed, the cabin temperature is significantly uncomfortable, and it would be un-desirable to turn off the PACS powered by APU, with already sweating passengers. Does this/can this happen without breaching SOP? What types could be capable of this?

The 777 packs are in AUTO mode thus allowing the aircraft (AIMS system) to leave the packs on or turn 1 or both packs off during engine starts if required.The Trent powered 777ER can dual start with no problem with the APU with packs in AUTO, however a 777LR can only start 1 engine at a time due to the required airflow to turn a GE90 regardless pf pack operation. I'm assuming the 787 has a similar system which enables dual start with CACs running/Auto but i havent worked on that aircraft whatsoever.

rjsampson wrote:

zeke wrote:

Hi didn't say they did, just indicated that one cannot run the packs and start the engines at the same time.

Is that due to the packs putting a load on the electrical system, sufficiently high to not crank the motor concurrently?

It's not an electrical thing, it's a bleed air thing.

Yes the APU does spin a generator that creates electricity to use before the engines are going, but it's second job is to provide compressed air for other systems... this air is routed through the pacs and cooled to create the cool cabin air. It is also used to start the spinning of the turbine as the aircraft engines are too large to be started with an electric starter.

thegman wrote:

rjsampson wrote:

zeke wrote:

Hi didn't say they did, just indicated that one cannot run the packs and start the engines at the same time.

Is that due to the packs putting a load on the electrical system, sufficiently high to not crank the motor concurrently?

It's not an electrical thing, it's a bleed air thing.

Yes the APU does spin a generator that creates electricity to use before the engines are going, but it's second job is to provide compressed air for other systems... this air is routed through the pacs and cooled to create the cool cabin air. It is also used to start the spinning of the turbine as the aircraft engines are too large to be started with an electric starter.

I thought 787 has electric packs and starters?

benbeny wrote:

thegman wrote:

rjsampson wrote:

Is that due to the packs putting a load on the electrical system, sufficiently high to not crank the motor concurrently?

It's not an electrical thing, it's a bleed air thing.

Yes the APU does spin a generator that creates electricity to use before the engines are going, but it's second job is to provide compressed air for other systems... this air is routed through the pacs and cooled to create the cool cabin air. It is also used to start the spinning of the turbine as the aircraft engines are too large to be started with an electric starter.

I thought 787 has electric packs and starters?

It does.

BravoOne wrote:

Can't recall at this moment but I believe we can start the 787 with the CACs running?

The B787 starters are electric. there is no bleed air starting

zeke wrote:

Hi didn't say they did, just indicated that one cannot run the packs and start the engines at the same time.

On the B787 it makes no difference, there's no real interface, I started the engines with the packs running when I went to training, the engines started just fine though I was looking for the APU to bog down when I did it. just due to the amperage draw. Tthe packs kept right on turning. amazing.. Never would have happened on a
Bleed air system. Almost as good as multiple engine starts on the B747-422 which I also tried in school. I never got to go to B777 run training, though I've heard the B777 can do multiple engine starts in an emergency. Can't think of what that emergency might be though.

strfyr51 wrote:

BravoOne wrote:

Can't recall at this moment but I believe we can start the 787 with the CACs running?

The B787 starters are electric. there is no bleed air starting

I have a rating in the airplane but have never flown it or even flown in one, but I do recall the starter gens. Not sure what that had to do with the original question?

On an airTran flight out of MSY on a 717, I was in 27A, and I am used to the A/C shutting off just before engine start, but while the air was still blowing me from the Gasper vents, the left engine rumbled and motored for a bit and then started. I was amazed. Amazed because as it is, the BR715 requires a LOT of air to get going.

I know the Avro RJ-85 could start up with the packs running, as the Honeywell LF-507 is a GTF, and doesn't require air to get started. This was the case on my Northwest Jetlink flight (Mesaba) on N519XJ

gtae07 wrote:

BravoOne wrote:

One cannot help but think that the Swissair MD11 accident would have benefited from a 3rd pilot to assist in what was an extremely difficult situation. Just because its legal does not mean its smart.

Wouldn't have done a thing. There was nothing the crew could have really done about the fire, and according to the Canadian TSB:

From any point along the Swissair Flight 111 flight path after the initial odour in the cockpit, the time required to complete an approach and landing to the Halifax International Airport would have exceeded the time available before the fire-related conditions in the aircraft cockpit would have precluded a safe landing.

Now, I'd argue that perhaps an immediate emergency descent would have been a better choice of action, even if in hindsight they couldn't have made it.

Same for Alaska 261. They spent too much time trying to troubleshoot. If they had immediately landed, they might have made it. If I were a pilot, first sign of trouble, I would find a runway. I would try to troubleshoot on my way to the runway, IF I HAD TIME. First priority, get the airplane on the ground, then figure out what went wrong.

On the 787, the pack Cabin Air Compressors are load shed off during engine start, either with the APU on or with all 3 external power cords connected.