The V1 conundrum
Years ago I was a deading jump seat pilot in a Boeing 737-200 that experienced a dual EPR blockage while taking off at night on a runway limited length of 5600 ft.
It transpired later that both Pt2 tubes serving the engines had been blocked well before take off because the engine covers had not been fitted during an overnight stop.
Looking back at that event years later both the captain and myself had the same sixth sense feeling at the start of the take off roll that the acceleration wasn't what we expected for a max thrust take off. Yet all the engine readings were normal at a quick glance. With four runway lights to go to the departure threshold it dawned on both the pilot and myself we were never going to be airborne by the end of the runway.
We were 15 knots below V1 by then and an abort would have been fatal due to no over-run safety area . There were boulders for a sea wall only 50 yards from the departure end which would have torn us to pieces had an abort taken place. When we saw that an abort would be fatal, the captain fire-walled both engines and pulled back hard on the control column into the black void of the night. Later FDR read outs showed the aircraft maintained 50-100 feet above the water for one mile before we climbed away. Airspeed was V1 minus 15 knots when the captain took those actions. In theory the 737 was runway limited for take off over the sea which is why the take off was 2.18 EPR bleeds off. The V speeds on the take off chart confirmed this - except we were not to know the EPR readings were faulty and the real EPR we experienced was more like 2.10 - well below the expected 2.18 EPR.
The N1 readings would have shown the actual power but there was barely 2mm difference between the needles of the expected 100% N1 and the 92% N1 the engines actually achieved. Being at night and with the instrument lighting turned low, the 2 mm difference was difficult to notice especially on both engines at the same time
Later investigation of the engine Pt2 sensors located at the bottom of the engine cowls showed both were blocked with a combination of phosphate dust from a nearby mine and insects.
While the crew including myself in the jump seat, could be blamed for not monitoring the engine instruments more closely (the lower than expected N1 readings was the main clue) the lack of acceleration feeling after brakes release was another missed opportunity. The actions of the captain when at the last few seconds he realised that we were never going to get airborne in the remaining runway length, was probably the best decision he had ever made in his flying career.
For my part, that experience made me have a long standing distrust of V1 as a stop/go decision point. I made a private promise to myself that in future I would make a decision to go on passing 15 knots below V1. My thinking was that the chances of a real V1 cocked up abort for whatever reason, was greater than the continue case. Fortunately I have long since retired knowing my theory will never be put to test.