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A long time ago I was on a 737 ( I believe -200 but it could been -300) with an inop APU and cross bleed. We taxied out on one engine and snuggled up behind another 737 - it looked like 15 feet from our nose to the tail of the other aircraft. The lead jet spooled up on one engine and after a couple minutes and lot of shaking our no starter engined fired off.

My question is is this possible and a modern high bypass engine?

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    $\begingroup$ This sounds highly improbable. Where were you sitting? $\endgroup$
    – John K
    Mar 6 at 15:03
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    $\begingroup$ I have a very hard time imagining a scenario where APU + the ability to cross-bleed start a motor are all deferred, but the aircraft can still be dispatched for revenue service. Perhaps the OP mistook the high power setting required for an X-bleed start (much higher in the -200 than in later models) for a buddy start. $\endgroup$
    – Ralph J
    Mar 6 at 16:14
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    $\begingroup$ MMEL has no mention of x-bleed valve so likely it cannot be deferred. $\endgroup$
    – vasin1987
    Mar 6 at 17:01

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We did buddy starts in the C-130 (practiced them more often than doing them operationally), so I'm familiar with the idea, and I've never heard any hint of the 737 having a comparable ability. For a -200 especially, with such a small fan, that sounds inconceivable. For a CFM engine, slightly less unbelievable, but to get the N2 where you need it... I strongly doubt that it could be done.

Even if it could work, sitting behind another jet blasting max takeoff power at your motor, it would be (1) outside of any published Boeing guidance, and therefore (2) reserved for a "last flight out of Saigon" scenario - not something to be done with revenue passengers on board.

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    $\begingroup$ I can see how a C-130 having a single spool constant speed turboshaft can be windmill started just by blowing air though the prop. A low by pass turbofan? I'd be pretty worried about FOD even if I could get the core spinning fast enough in the jet blast of the airplane ahead, and that still seems pretty implausible. $\endgroup$
    – John K
    Mar 6 at 18:25
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    $\begingroup$ Chuck Yeager's autobiography describes buddy starting single engine jet fighters in 1950s German as a 'WWIII has started' contingency plan, but being Sabres this would have been much better geometry for this trick than the described 737, and the autobiogrpahy can be described as 'colorful' so needs to be treated with care as a source. $\endgroup$ Mar 7 at 10:35
  • $\begingroup$ Yeah you could nose the airplane right into the other's tail pipe and get enough flow to windmill the back one's engine, and only a minority of the mass flow is burned so there is still plenty of oxygen. $\endgroup$
    – John K
    Mar 7 at 19:53
  • $\begingroup$ For the doubtful, it was a 737 and the Captain even briefed us passengers on the procedure that was about to happen. i’m in the industry so I was fascinated. Again, I believe it was a late model 200 series. I too am skeptical that modern high bypass engine could be buddy started. I can’t imagine enough air could be forced down the core to bring it up to speed but I thought I would ask. $\endgroup$
    – Tangojul
    Mar 9 at 3:27
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In order to start a turbofan engine, you need to spool up the core (HPC Compressor and HP Turbine) to a high enough speed to provide airflow of enough volume and pressure to sustain combustion when fuel is introduced to the combustor. You might be able to get the low pressure rotors (fan and low pressure turbine) to rotate, but not the core to the extent to enable combustion.

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