From time to time, the CAA informs instructors and flying schools of topics which deserve additional attention. A recent one is ‘Partial Power Loss’.
We’ve all practiced EFATO’s and PFL’s, with an abrupt and total loss of power. Partial power loss is much more insidious. It’s easy to accept a poor climb rate, without realising what is happening, and there is always the temptation to keep pitching up to maintain a climb, resulting in a power-on stall.
There can be many reasons for partial power (e.g. plugs fouled, one magneto failed, fuel flow restriction, carb ice, failure of a constant-speed propeller mechanism etc etc) and they can occur at various altitudes. So there is no ‘one size fits all’ solution.
As general rules
- maintain an airspeed not lower than best glide speed, or not less than Vx with obstacles to clear
- understand what rpm in your aircraft is sufficient to maintain altitude
- be ready for total power loss.
If you suffer partial power loss, first ask the question can I still climb? If so, climb to a safe altitude, and if close to the airfield, fly a close-in circuit and land. Don’t hesitate to make a PAN or MAYDAY call. If partial power occurs on a cross country route, and you can maintain altitude, them proceed cautiously to the nearest airfield and seek a straight in approach. Of course, I’d be looking for fields along the way, in case the engine fails. If you are not maintaining altitude, then a forced landing is inevitable, but the residual power gives you a little bit more choice and flexibility of location.
The ATSB in Australia have published a good document on the subject. It’s worth a read and can help think through some scenarios for our own aircraft. You may wish to try some of these with an instructor.