The deadly EPR

1757
Aircraft instrument panel
image: Duncan Grant | grantdesign.com.au

John Laming on the dangers of incorrect instrument readings during take-off

Your first instructor may have told you to keep an eye on the engine temperatures and pressures during take-off.

The reality is that oil and cylinder head temperatures are slow to indicate trouble, and oil pressure needles can flicker and vary by small amounts depending on engine power and RPM. A sudden drop of oil pressure near lift-off could be an impending engine failure or a gauge malfunction but there is no way of knowing for certain. A late abort for an engine gauge reading has the potential for an overrun accident.

In December 1973 a Lockheed Hudson was taking off from Horn Island, Queensland, when the pilot noticed fluctuations in the oil pressure gauge for the left engine. He closed both throttles and abandoned the take-off. Despite heavy braking, the aircraft overran the runway and ran into a creek. No malfunction was found in the port engine or its oil pressure indicating system.

The correct time for a last-minute assessment of engine health is at brake release, with priority then given to directional control, lookout and airspeed indications.

Good airmanship dictates that a critical stop/go decision should not normally be based on one gauge alone. It is wise to check for corroborative evidence before committing to what may prove to be an irrevocable course of action.

In January 1982, an Air Florida Boeing 737 at Washington, DC, having lifted off from the snow-covered runway, was unable maintain altitude. It stalled and, after hitting a bridge (and killing several people in cars), crashed nose first into the frozen Potomac River. The black box recorder showed the engines had not delivered full power during take-off.

The engine power indicators had given false information to the crew, possibly due to ice blocking air inlet tubes which in turn, sensed the power delivered. Known as Pt2 sensing tubes, they show the engine pressure ratio (EPR) on a cockpit gauge. The advantage of the EPR gauge is that accurate power settings can be measured, when the system works as advertised.

But if the front tube is blocked, the sensor thinks no air is coming into the front of the engine. The rear sensor, operating normally, senses lots of high-pressure hot air being ejected from the tailpipe and thus, the EPR will overread. The natural tendency to remedy the apparent over-power indication on the EPR gauges is for the pilot to ease the throttles back to keep within perceived engine limits. However, the engine RPM gauge will show the pilot the real power being produced.

After the Potomac accident, notices were sent to all operators of Pratt & Whitney JT8D series engines, warning crews to be on alert for erroneous EPR indications in icing conditions and to rely primarily on the engine RPM gauge for actual indications of power. Typically, the RPM gauge is called an N1 or fan gauge and, depending on the type of engine, indicates 95 to 101 per cent on take-off.

The following incident occurred on a Pacific atoll where a seemingly implausible combination of factors caused a Boeing 737 to come just a few seconds away from disaster. The length of the runway was 1700 metres, with a sea wall at the departure end. I was deadheading in the jump seat and had a front-row view.

Lined up for a night take-off, the captain opened the throttles to 1.6 EPR on the brakes, checked that both engines spooled up evenly, then advanced the throttles to the planned take-off power of 2.18 EPR. Apart from the standard 80-knot airspeed comparison check, company SOP required no calls were to be made by either pilot unless something was seriously amiss.

On this occasion, the take-off seemed to proceed normally, although the rate of acceleration was somewhat slower than expected. It wasn’t until the aircraft had reached the expected lift-off point down the runway, that the captain realised the take-off run was longer than planned and, unless he took immediate action, the aircraft may not get airborne before the end of the runway.

An abort at that late stage was out of the question. With that, he ‘firewalled’ both throttles to their maximum stops and hauled back on the control column.

The last runway light disappeared under us, as did a fleeting close-up sight of the blue flashing strobe light of a shocked traffic cop’s motorbike. I felt the reassuring surge of thrust propel the 737 upwards at a deck angle of 20 degrees and silently thanked the Lord that the captain had made an instant correct decision to firewall those Pratt & Whitneys. I knew the Potomac accident might have been averted if the crew had only pushed the throttles to the stops.

Later analysis of the flight data recorder showed the aircraft had lifted off at 15 knots below the calculated VR or rotation speed and we had flown just 19 feet above the sea for a short distance before gradually climbing away.

The seemingly implausible had occurred in the form of a simultaneous false reading on both EPR gauges.

After flap retraction and the aircraft had settled into a 250-knot climb, there was time for a detailed scan of the engine instruments. Compared to book figures, the N1 indications seemed low when compared with the 1.93 EPR climb setting.

The seemingly implausible had occurred in the form of a simultaneous false reading on both EPR gauges. There was nothing wrong with the engines. The decision was made to return to land. The landing lights on short final illuminated drifting mists of phosphate dust from a nearby mine.

As the passengers disembarked to wait out the delay in the airport terminal, engineers had already removed the engine nose cones to check out the Pt2 system. They discovered the Pt2 tube of each engine was blocked with congealed phosphate dust and some other glutinous substance. Normally, a cover was placed over the front of each engine during overnight parking. On this occasion, the covers had not been used.

The gauge scale between 91 per cent and 100 per cent N1 is less than 3 mm and difficult to read in low light, especially at a quick glance. This might explain why the crew failed to notice the apparent lower than normal N1 readings on the take-off run. This incident also demonstrated that at night, it is difficult to make any meaningful correlation between rate of acceleration and runway remaining.

Later versions of the Boeing 737 have CFM56 engines which rely on N1 gauges as the primary power indication. EPR gauges still remain on many older jet transports.

2 COMMENTS

  1. The Pacific Atoll incident result had a successful ending due to an experienced pilot making a quick and correct decision. In this case, hesitation would have resulted in disaster.
    Job well done :-)

  2. Another excellent way to cross check and this works on virtually all transport category Jets is to check that 80knots is attained within a maximum 20 seconds acceleration time.

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