Slow acceleration on a wet approach at Rotorua
Rotorua (NZRO) runway 36, 1,700 m, wet. Twin turbofan, in cloud with moderate icing, engine anti-ice on, both engines at flight idle for 18 minutes during the descent. At 5 nm final, 2,500 ft, 160 kt, the PF advances the thrust levers towards 70 % N1. The right engine reaches 65 % N1 in 8 s. The left engine takes 14 s and its EGT rises about 45 °C higher than the right before both settle at the commanded N1. The left engine accelerated normally on the previous sector. Ceiling 900 ft, visibility 4 km in rain, wind 340/12 kt. There is no engineering support at Rotorua and the aircraft is scheduled to night-stop there.
1. What are the credible causes of the slow acceleration, and which can you test in flight?
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Credible causes, roughly in order of likelihood on a cold, icing day:
- Low idle and cold soak. After 18 minutes at flight idle in icing, the engine is at minimum idle and the acceleration schedule is working from the bottom of its range. Both engines should behave similarly, but the one carrying more bleed demand or sitting slightly lower in idle will lag.
- Bleed configuration. Engine anti-ice on both engines plus packs; a bleed valve that is slow to modulate or stuck partially open leaves that engine with less air for acceleration and a hotter EGT.
- FADEC/fuel control scheduling - a degraded EEC, a fuel metering unit slow to respond, or a filter approaching bypass.
- Compressor condition - fouling, tip clearance, variable stator vane scheduling. A worn compressor accelerates sluggishly and runs hotter.
- Sensor error - possible, but a 45 °C EGT split is real heat, not just a display artefact.
What you can check in flight: both engines at the same N1 and identical bleed configuration; whether the split persists at higher power; whether the EGT difference tracks the N1 difference (real engine issue) or not (sensor); and whether the lag repeats on a second thrust change. You cannot fix it in flight, and one sample is not a trend.
2. Do you land, go around, or hold? Justify it with performance and handling considerations.
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Land - but manage the risk that the go-around you might need is the manoeuvre this engine is worst at.
The scenario is poor for a go-around: 900 ft ceiling, 4 km visibility, a wet 1,700 m runway, and an engine that needs 14 s to accelerate from idle. A go-around from idle leans on the slow engine; a go-around with both engines already spooled does not.
Practical plan:
- keep both engines at or above roughly 60–65 % N1 from 5 nm final until touchdown, so any acceleration is a small one from a running engine, not a start from idle;
- brief that in a go-around the good engine is advanced first and yaw is expected, with the slow engine following;
- add a speed increment for the wet runway and the wind (Vref + 5);
- if the left engine fails to respond at all, you are effectively single-engine at low power - land straight ahead rather than manoeuvre;
- do not hold to 'see if it clears'. Ten more minutes will not fix a bleed valve or a compressor, and the weather is not improving.
On a 1,700 m wet runway with a 900 ft ceiling, the decision to keep the engines spooled is worth more than any derate you saved on the approach.
3. What goes in the technical log, and what must be checked before the next flight?
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Log entry, with numbers: 'L ENG slow acceleration: approx 14 s from flight idle to 65 % N1, versus 8 s on R ENG; L EGT transiently ~45 °C above R before stabilising at commanded N1. Icing conditions, engine anti-ice on, packs on. Previous sector normal.' Include altitude, OAT and phase of flight so the engineer can relate the event to the conditions.
Before the next flight, engineering should:
- read the FADEC/EEC fault codes and any trend or exceedance data;
- check bleed valve and anti-ice valve operation, and fuel filter delta pressure;
- check fuel control/EEC rigging, and inspect the compressor if the codes point that way;
- reproduce the acceleration on the ground in similar conditions.
If the lag and the EGT split repeat on the ground, the aircraft does not go. A slow-accelerating engine is often an early sign of compressor damage, a sticking bleed valve or a fuel scheduling fault, and the next manifestation can be a surge or a failure to accelerate at the worst possible moment - a go-around, or a wave-off on a wet runway.