Initially posted by HPSOV
Remeber that 737 that had ice on its engine probes and went into the Plotomac (spelling??). Those guys set the required EPR but due to faulty indications didn't have anywhere near the required thrust.
On that airplane the N1 indicators were at the top of the engine instrument stack with EPR being the second one down. When power was applied for takeoff, the pilots referenced the EPR gauges and read what appeared to be the proper EPR, and we know now that it was not correct (because the PT2 probes were clogged, probably with ice – but NOT from what a lot of people believe), and scanning down the instrument stack from that point, as is the habit with most pilots, all the other indications appeared to be “normal” (unfortunately, at that time, normal was essentially understood to be having the needles of both sets of gauges parallel with each other) and the specific N1 indication, which could have been seen as quite low, was not taken into the scan – or at least that is the speculation. However, each engine was producing about 75% of the power that should have been used – 25% less than expected. The piece that goes unnoticed by many observers was that the airplane was, and still is today, certificated to take off with a 50% loss in power. By that relationship, the airplane had 50% more power than it should have needed to get safely airborne.