B737-low N1 during take off roll could mean trouble. .

[Centaurus]

The guys who put the 737 into the Potomac didn't put the throttles full forward ever......

Readers of that accident may recall that ice had blocked the Pt2 sensors. In turn, this caused the EPR gauges to over-read. The pilots did not notice that the N1 indication was significantly lower than rated take off thrust because the pilots had set planned takeoff EPR unaware the EPR readings were erroneous. In those days on the P&W JT8D engines, EPR was the primary means of setting take off power despite EPR being prone to erroneous indications under certain weather conditions.
See: https://en.wikipedia.org/wiki/Air_Florida_Flight_90.

Comments have been made about the lack of action by the crew of the Sunwing 737-800 which departed from Belfast in day VMC. Some readers were of the opinion that the crew should have noticed the lack of acceleration associated with the low N1 and done something about it. Maybe so.

This writer was involved with a similar incident on Nauru in the 1980's where erroneous EPR indications caused by partially blocked PT2 sensors caused both engines EPR to over-read simultaneously. The take off was at night and the runway length 5600 feet with no overrun safety area. The runway end finished up in a boulder strewn small cliff leading to a drop into the sea.The three pilots up front failed to pick the initially slow acceleration in the early part of the take off roll; possibly because the slight reduction of power was equivalent to a normal reduced thrust rate of acceleration. If the runway had been longer the incident would not have been immediately apparent.

But a ten percent reduction of thrust (N1) on a limited length runway, coupled with seemingly correct EPR (erroneous indication as it transpired) was potentially deadly, as it nearly turned out.During the whole take off roll, the engine instruments (EPR, N1, fuel flow, engine oil temps and pressures) showed parallel needle indications as both engines were performing exactly as expected. Both EPR needles were 2.18 EPR corresponding to max bleeds off thrust. Due to the dim lighting required for the night take off, the difference between the expected 100 percent N1 and the actual 90 percent N1 shown on the gauges, was about 6 mm or less than the width of a needle and needed more than just a quick glance.

It wasn't until there were about five runway lights to go did it become apparent that VR was not going to be reached before the end of the runway. At VR minus 15 knots the captain took control from the PF and simultaneously "firewalled" the throttles and rotated early to get airborne before the runway end. Any late abort (we were still below V1) would have been disastrous and we would have gone off the cliff.

The point that stuck in my mind is that no one picked the less than expected acceleration, either early in the take off roll or later in the take off roll, until it suddenly dawned that with only a few runway lights to go we were never going to get airborne by the end of the runway. . Yet, throughout the take off run, all the engine instruments including the vital EPR gauges showed normal settings. They included the N1 needles; except closer scrutiny during the take off roll would have revealed slightly lower N1 than expected and planned. It was because all gauges looked normal throughout he take off roll. In hindsight, it was all too easy to blame the crew for not picking the small difference in N1 from the planned setting..

With the same operator there had been two earlier instances when the PT2 sensor on one engine was blocked by foreign objects. In that, case one EPR needle was seen to suddenly increase its reading well beyond the planned maximum setting and the split between the two EPR's were quickly noticed by the time the aircraft reached 80 knots. At 100 knots the F/O tried to throttle back that engine to equalise the EPR readings but was prevented by the captain who directed the F/O to set equal N1 and ignore the faulty EPR reading. But in the incident described earlier, there was no split between the EPR or the N1 gauges because both PT2 sensors of the engines were blocked and thus nothing obvious to a quick glance during the take off roll was noticed. The blockage of the Pt2 sensors was caused by a combination of phosphate dust from the nearby mine and insects that had nested in the tube sensors.

The point of this story is this: At night especially, degraded aircraft acceleration (for whatever reason) may not be immediately apparent until the departure end of the runway is approaching fast and it becomes obvious the aircraft is committed to go; even if it means early rotation and fire-walling the throttles.