Saturday, November 27, 2010

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.


  1. very interesting, so does this means that you have to manage perfectly your potential and kinetic energy in order to get to the field without spooling the engines up again ? If yes, do you plan your descent or it's by feeling only ? great to read that you are going to fly a new airplane !

  2. Hey, well its not really a 'new' aeroplane! its still made in the 1980's, just a new sort for my logbook, thats bigger, sounds mean, and has more power!

    As for the descent, i spoke about it in an earlier post. We use a descent profile of 5 which means my height times by 5 is my descent point. so at 9000ft, i use 45nm as a rough guide to plan my descent. 19 out of 20 times it works beautifully but sometimes it bites me.

    But yes the aim of the game in pistons is to not increase power again, unless its absolutely necessary for operational reasons.


  3. thanks, I should have said another airplane that's actually what I meant :D . 5x your height, that's pretty steep isn't it ? Hope the gliding performances are better just in case ;-)

  4. So does the 'no added power' include final approach, say, if you found yourself a little below glideslope? What about if you need to level out for a downwind join or some such? Or would you sacrifice a bit of airspeed in those situations in order to keep the power reductions going all the way until you're at idle for the flare?

    Really interesting post all in all Mike, I'm going to start trying to fly a bit more professionally and precise along these lines - what with my CPL test in 2ish months, it's probably a good idea!

  5. 5x the height at 500ft per minute, so 45nm at 9000ft should get you to a good height by the cct, or a straight in approach, wind depending.

    and brendan, thanks for reading. But in the 400 series cessna's, we don't increase the power pretty much at all.. we use the flaps and gear to control the airspeed and then reduce the power as we are stabilised on the approach, usually at 500ft. Thats full flaps, pitch full fine and gear down.

    ive had to increase powers on barons and singles, mainly due to inexperience and also wind shear etc, sometimes its unavoidable obviously, but the name of the game in the piston world is a constant reduction of power.

    hope that makes sense


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  7. and to answer that a little bit better, in an ILS in the 402, i keep the powers at 25", gear down and first stage of flap. in the 402C its a lot easier, but the B gear extention speed is 140kt. plus we are operating at CATB speeds, so technically, we need to be at 130kts after the outer marker which is the final approach fix. some people like you to be 130kt by the interception of the localiser and the glideslope.

    as for downwind, thats usually where we are trying to wash off speed anyway, and going from say 160kts down to around 130kts, so its another method of bleeding/gaining speed without touching the engines.

    thanks for reading

  8. Mike: I know that this response is to an OLD post, but I hope you see it. (I'm a new follower, cruising through the archives...) I enjoyed your discussion of engine managment on descent. Very detailed and healthy for the engines. I have to wonder what you would do in a busy, high-traffic area . The slow taxi, rather than just sitting, to enable the necessary cooling, is brilliant!! We all learn something new, every day! Thanks. -C.