Engine Management and Cost

I have a few things i want to write about soon, but i thought i would go through some piston engine management, and also how we try to operate aircraft, both efficiently and operationally to try get the best results without compromising maintenance times.

I'll give a little run down into how it costing works firstly. Lets for an example use a 402. I'm actually unsure as to what my company charges per hour for their aircraft. As line pilots we just do the flying, and leave the operations and bookings to the operations staff. However, for example lets say it costs $800 per hour for a 402. Then for a flight that should hypothetically take 2 hours on the maintenance release, then costing should be $1600. So the aim of the line pilot is to try do it within that 2 hours. Usually it's pretty spot on, but sometimes other variables means you will go over. Such things as holding, or weather deviations, a go around or instrument approach etc are all things that will increase your maintenance times. Maintenance times are done in 6 minute intervals, so therefore an extra 6 minutes of flight time will equal an extra .1 of an hour to the maintenance release. When we consider that its $800 and hour, then every .1 will equal $80 saved or spent. Therefore as an aircraft operator, i do what i can to increase efficiency and do what i can to reduce my flight times.

I do this by usually doing straight in approaches where i can. Nearly every runway is east-west up here, and the wind is nearly always from the east. Most of these runways are long enough, and sealed, that even a slight tailwind doesn't pose any real problem. Unfamiliar strips, or dubious dirt strips always require an inspection first. There is a comprimise of safety and efficiency, which is where basic airmanship comes into play. I do other things such as climbing or descending to different levels to see where the best winds are. Usually going east, the winds are lighter up higher, and heading west, a lower altitude gets you the best groundspeed. The most efficient way of working out winds is talking to already departed company aircraft and seeing what groundspeed they are getting at a level, and levelling off at a lower or higher altitude and compare. Other ways of reducing track miles is requesting direct to waypoints or aerodromes. This is done frequently when we have to divert left and right of track, it becomes more efficient to track direct to the next waypoint than manoeuvre back on the original track.

So with all this in mind, engine management for piston aeroplanes plays a big factor. Big piston engines are not like turbines and you cannot just pull back the taps to slow down. There is a constant reduction of power, till you retard the throttles on landing. The aim of the game is after reducing from take-off power to climb power is that you don't actually increase the powers again, unless in an event, such as a go-around requires you to do so. This was the same from flying C206 all the way up to the C402.

Without turbo engines to deal with, the normal range on normally aspirated engines is 20 - 25" of manifold pressure that needs to be looked after. To reduce the power too quickly especially on descent will cause shock cooling which will eventually crack the cylinder heads of the piston engine. So to avoid this, and look after the engine, most companies have a standard operation procedure on how they want their engines operated and power reduced when descending. On normally aspirated engines, we keep them full throttle till around 6000ft, which equals roughly 23" of manifold pressure. We keep them at 23" which is about an inch of power every 500ft - 1000ft worth of descent. This equates to an inch of manifold pressure every 1 - 2 minutes. When we are through 3000ft we reduce it to 21" of manifold pressure and reduce it again through 2000ft, back to 19". From there it is pilots discretion to keep the constant rate of descent and power reduction smooth and consistant till retarding the powers on landing.

This gives you a rough idea of how we operated the singles and baron's. Using this method we avoid shock cooling the engines, and also keep our speed up until near an aerodrome, which helps reduce flight times, without being detrimental to the aircraft engines.

So i will now talk about the engine management in the turbo engines. It is much the same philosophy as the normally aspirated piston, but with a much higher manifold pressure. The C402 cruises at 29" of manifold pressure. So we keep a constant rate of descent of 500ft. When we go through 3000ft, we reduce the power 2 inches, back to 27". Through 2000ft, we reduce it back to 25". This is actually where the turbos cut out, so its essentially back to a normally aspirated engine at this point. It is still important to give it a minute or two at 25" though for adequate cooling and adjusting. (on this note, if your descent rate happens to be higher, either to catch profile, or because you had to descent later, then obviously you would reduce the powers a little earlier to keep the 2 minute spacing between power changes.) In the C402 we keep the powers at 25" until we are gear down, full flap, pitch full fine, mixture full, and then start reducing the powers till retarding them for landing. Once we have landed, we also need to allow a minimum of 3 minutes for the turbo engines to cool on landing. For this reason i nearly always use the entire runway and backtrack (when operationally available) so during the cooling phase the aeroplane is always moving. I have discovered passengers hate sitting in a stationary aeroplane with the engines running, but if im taxiing around after landing no one really minds. So i always try use the entire 3 minutes just backtracking and vacating the runway. Doesn't always work, and in Darwin the taxi to the General Aviation ramp is about 4 minutes, so in some cases it doesn't pose a huge problem.

Hopefully this makes sense to all you readers. If you have any questions please ask, just a little insight into how i have been taught, and am expected to operated company aircraft. Im actually starting on the C404 soon, which is a little different with engine management from the C402, as it cruises at 31" and has turbo, geared engines, which are quite sensitive to shock cooling and have to be looked after more diligently than the C402.


Thanks for reading.