With 1 engine operating, what Ng must be maintained to crossbleed start the other?

With 1 engine operating, what Ng must be maintained to crossbleed start the other?

• A. 100% NG
• B. 70-80% NG
• C. <50% NG
• D. >90% NG

Answer: (D)

Cross-bleed starting relies on the running engine supplying stable bleed air to start the other engine. To ensure that bleed air is enough to drive the starting system and reliably spool up the second engine, the operating engine must be kept at a high gas generator speed—that is, above 90% Ng. When Ng is above this threshold, the bleed pressure and flow are sufficient to turn on the starter and bring the other engine to light without surges or stalls. If Ng is too low, the bleed source is inadequate, and the cross-bleed start can fail or be unstable. So the rule of thumb is to maintain Ng greater than 90% during a cross-bleed start.

Cross-bleed starting relies on the running engine supplying stable bleed air to start the other engine. To ensure that bleed air is enough to drive the starting system and reliably spool up the second engine, the operating engine must be kept at a high gas generator speed—that is, above 90% Ng. When Ng is above this threshold, the bleed pressure and flow are sufficient to turn on the starter and bring the other engine to light without surges or stalls. If Ng is too low, the bleed source is inadequate, and the cross-bleed start can fail or be unstable. So the rule of thumb is to maintain Ng greater than 90% during a cross-bleed start.