The uncertainity factor in stop or go situations is often critical. Particularly if the runway length is marginal and it is night time.
For example, I was jump seating when a Boeing 737-200 took off on a performance limited runway at night. That means a rated thrust bleeds of takeoff which usually means a real kick in the back at full power. At the far end of the runway was a road, then large boulders that formed a sea wall with a drop of 20 feet into the Pacific ocean. A motor bike policeman was stationed on the road to stop traffic until the airraft was airborne.
What we (the captain, the first officer and me as a pilot observer) did not know was that insects and blowing phosphate dust from a nearby phosphate mining area had already blocked the entrance to the Pt2 sensor situated at the front of the P&W JTD-17 engines. So when the first officer applied power to achieve 2.18 EPR at 101% N1 the captain took over the thrust levers and set 2.18 EPR. The engine instruments of N1 and N2 are quite small and the instrument lighting was set low because of the sheer black Pacific night.
In fact, although the EPR gauges showed the needles on 2.18 it turned out to be in this case, a false reading similar to the iced up Pt2 tubes of the Air Florida 737-200 that crashed into the Potomac River at Washington.
Passing one third of the way down the 5200 ft runway of this Pacific atoll, I had an uneasy feeling that the acceleration wasn't the kick in the back that I had been expecting. I leaned forward to get a better focus on the N1 gauges to confirm their readings agreed with the expected 2.18 EPR. I was not wearing my glasses at the time and it was difficult to see the exact reading of the needles of the N1 gauges because of the small graduations. The difference between 101% N1 and 90% N1 was about 4 millimetres.
As I was focusing on the readings of the N1 gauges, I noticed the captain was rapidly glancing ahead at the runway remaing then back at the instrument panel. I wondered if he had the same feeling of unease as me. With the end of the runway coming up fast, it suddenly dawned on me that we were never going to become airborne before the end of the runway - especially as the airspeed was was still approaching V1 with three runway lights to go. At the same time I realised we were in dire straits, the captain acted fast and taking over control from the first officer he firewalled the thrust levers forward against the mechanical stop and hauled back on the control column. . An abort at that point would have been fatal. There was an immediate surge in power as the EPR shot up to 2,30 EPR or thereabouts. . We cleared the threshold and the captain was immediately on instruments. What we didn't know was the jet blast blew parts of the road and sea wall boulders back over the threshold and along the runway. It had been a very close shave. . The policeman blinded by the aircraft landing lights coming closer had already seen the Boeing bearing down on him like an express train and had gunned his motor bike away from danger fast.
At a safe height the captain set standard 1.94 EPR climb power on both engines and commenced flap retraction. During the climb and after the flaps and leading ege devices had fully retracted, we accelerated to 280 knots climb speed. The rate of climb at 10,000 ft was much less than expected. A comparison of climb EPR versus N1 from the manual revealed the N1 was lower than expected;.as was the rate of climb. On a takeoffa few weeks earlier, I had experienced the effect of a single blocked Pt2 sensor which had cleared itself after application of engine anti-ice; even though the OAT was quite high at 25 C.
A simultaneous blockage of both Pt2 sensors at the sea level temperature of 30 C was something I had never heard of before. With both engine readings showing parallel to each other (including EPR and N1) during the takeoff and climb, it was not immediately obvious that we were experiencing a double Pt2 sensor blockage unless one was quick enough to take the N1 readings. Keeping in mind it was night and instrument lighting low.
The captain decided to return to land as the cause of the low rate of climb was not obvious at that point. Fuel contamination was suspected since both engines had identical readings. After touch down, normally 1.6 EPR is used for reverse thrust. I hinted to the captain to use full reverse against the aft stop in case the EPR was mis-reading because at least we knew we were obtaining full reverse. The landing was normal; as was the taxy in. On the ground, the cowls were removed and evidence found of insects and phosphate deposits in both Pt 2 sensors. Both sensors were totally blocked. Hence the subsequent EPR faulty reading. The engineers had forgotten to install covers over the engine intakes during an overnight stop.
Calculations made after the event revealed the actual EPR attained during take off was around 2.07 although both EPR gauges showed 2.18 EPR. A closer check of the N1 during the takeoff would have revealed a figure of around 90% on both engines instead of 101.5%. The fact that all engine readings were parallel to one another led us to believe that all was normal.
On the other hand, if only one Pt2 sensor was blocked it would have stood out as a significant split between the two sets of engine readings and thus the crew would have been alerted at the beginning of the takeoff roll and rejected the takeoff.at low speed. The lesson I took from that event was the difficulty of assessing remaing runway length to go at night and to use N1 as the prime thrust setting and not EPR if fitted.
Last edited by Centaurus; 2nd Feb 2021 at 09:21.