Ng (Engine Gas Generator Speed) and Fuel Flow Limiting

It is alarming for MH60T operators to exceed power available from a T700 series engine, prior to reaching TGT limiting or any other engine or torque limitation. Ng limiting of engine power starts to occur between 0 to -10°C (temperature varies due to altitude and MTA or ITA). At altitude, in low temperatures where maximum power available is below a 10 second torque timer, pilots describe being under torque limits (no timers), under TGT limits (in the green or yellow 30-minute timer), under Ng limits (no timers or a 30-minute timer), but exceeding power available with decreasing Nr (drooping terms). This is puzzling to crews who have not been introduced to Ng and fuel flow limiting. 

The power available charts (MTA, ITA, and CTA) show power available peaking and then decreasing around -10°C. The point where the limiting factor changes from TGT to Ng is illustrated on the charts where power available slants down to the left.  Simply put, in very cold temperatures and at high altitudes, pilots can have all engine indications under timed limits (TGT, Ng, and Torque), but run out of power. Although accounted for in the power available charts, unlike most operations where timers and “red” engine indications/tapes help crews identify when they are approaching and exceeding power available, Ng limiting in cold temperatures at altitude (when power available is less than the 10 second torque timer) has no indicators or warnings. 

For further explanation, below is a summary of several services’ doctrine: 

Mach airflow through the engine causes engine roughness, engine surge, and/or compressor stall. Because critical Mach speed decreases with colder air, the colder the air, the more restrictive the engine must be on Ng speed to prevent airflow from reaching Mach speed. Ng limiting restricts fuel flow (via Ng governing or max fuel flow) to control the rotational speed of the compressor/gas generator turbine rotors (based on turbine inlet temperature – T2). 

To be thorough, I have included fuel flow as a limiting factor because there are narrow windows at cold temperatures where the hydromechanical control unit (HMU) is “max fuel flow limited,” meaning that the fuel metering valve is wide open. This can occur just prior to Ng limiting as power available decreases on the left side of the power available charts. I have found no operational advantage to discerning whether Ng or max fuel flow is the limiting factor. Rather, the important point to remember is that there is another limit to power separate from TGT.  

We have focused on Ng limiting, a cold weather condition where cold, dense air reaches Mach within the compressor section of the engine. In contrast, Ng timed limits are a function of component life and should not be confused with actual engine environmental limitations. The warmer the temperature, the faster Ng is permitted. The maximum normal Ng is around 103%. As the MH60T’s engines have become more efficient (produced the same amount of torque at lower TGT) throughout my flying career, the Ng 12 second limit above 102% has increasingly become the limiting factor of the engine at warmer temperatures near sea level. Being Ng limited in cold temperatures, due to Ng governing or max fuel flow, is different than reaching timed Ng limitations in warmer temperatures because there is no cockpit indication (tape color or timers) alerting the crew they are approaching power limits in temperatures below -10°C.