apu空中无法启动，速度低于风转启动的速度的话，如何重新启动引擎呢？？
呵呵～这个问题困扰为了好长时间
手册上说的：
Air from the bleed air duct powers the starter motor, which is connected to the N2 rotor. The start air source is normally the APU, but air from ground cart or another running engine can be used.
后面关于APU说了：
The APU can be started on ground and left running for take off can be operated in flight up to 20 000ft
这么来说，747的所有引擎如果空中熄火，速度还不够风转启动的话，就没法再启动一个引擎喽，但那个 british airway 9又是怎么启动的引擎呢～