They have variable geometry inlet guide vanes on the fan and compressor, and continuous bypass from the fan to the augmentation section. Of the compressor's seven stages, the first three have variable stators. A single-stage low pressure turbine drives the fan, while a single-stage high-pressure turbine drives the 7-stage axial flow high-pressure compressor. Overall, the F404 engine is fairly simple, with relatively few moving parts. Compared to other recent turbofans, the F404 has experienced rather few developmental problems. In particular, it demonstrates high resistance to compressor stalls, even at high alpha. If an unlikely stall should occur, engine and afterburner relight is automatic.
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Engine response is remarkable, with spool-up from idle to full afterburner in just three to four seconds. However, acceleration time from Mach 0.8 to Mach 1.6 was originally longer than the required. Although some progress has been made in improving this response time, the problem has persisted in spite of numerous attempts to fix it. An Inflight Engine Condition Monitoring System (IECMS) continuously monitors the engine for critical malfunctions and parts life use.
