Friday, November 8, 2013

Mechanic Geek Talk on the Continental IO-550-F and -N Engines

Since I'm getting SO much great feedback about all the technical aviation stuff (note sarcasm), I thought I'd post some great info regarding how things are going with a modification that we have done to our aircraft (Cessna 207)- installation of a Continental IO-550-F 300 horsepower engine.

Our new pilot-mechanic, Rob, has recently interacted with a couple other pilots, (one in Africa; one in South America) and has come up with some great insights to this engine installation and operation.

The dialogue is representative of how the mission aviation community helps one another out and really sees one another as colleagues in being the hands and feet of God.  Enjoy!

Submitted by Jeremiah Diedrich SIL Aviation Porto Velho, Brazil

I'll share with you what we have learned in 250 hours of IO-550N flying on our 206 and 500 hours of IO-550N on the Asas de Socorro float 206. 

PERFORMANCE TESTS & CHARTS:  One of the things that we have done to measure our performance is to do some test flights with the...

...aircraft loaded to exact weights and a ground crew measuring exact takeoff distance and initial climb angle. I have attached the chart of the takeoff distances that we got and they have proven to be accurate or even a little conservative because of the slight tail wind we had on the day we did the testing.  The takeoff chart shows meters of ground roll on the vertical axis and aircraft weight in kilos on the horizontal axis.  Testing was done on a grass runway with a 3000' density altitude using 25deg. flaps with Robertson STOL, Flint tip tanks and a rotation speed of 43kts.  Vy climb angles were +9deg at 1200kg and 6deg at 1633kg holding 55kts with 25deg of flaps. Keep in mind these numbers were with a 3-5kt tail wind on climb out.  From what we have seen gross weight (3600) Vy at 3000' DA gives 1000 fpm climb rate or better in no wind situations.  Based on these numbers we have decided to increase our takeoff weights by 50kg or up to 1633kg (3600lb) whichever is less on our weight limited runways and then fly at these new limits for several hundred hours to "prove" our calculations before considering any further takeoff weight increases. 

LEAN OF PEAK RESULTS:  We have been flying lean of peak EGT after the first 100hrs of break in flying.  The procedure we use from sea level up to 9500' is upon reaching cruising altitude to set 75% power based on book figures for MAP and RPM and then reduce fuel flow to 48 liters per hour or about 12.5 gph.  Using the formula of 14.9 x fuel flow in gph (12.5) = horsepower we get an output of 186hp which equals 60% of rated power.  Above 9500' 48lph does not keep all cylinders lean of peak so we set wide open throttle and 2550rpm and then lean until the last cylinder peaks and run there or a bit leaner.  These power settings are giving us about 132ktas at 6500' and a little slower lower and faster higher. 

BEST RANGE & ENDURANCE:  I am still trying to work out best range and best endurance numbers since they are a combo of engine, prop and aircraft there is no formula that I have found.  I did on one flight at 7500' set the MAP and RPM for 60% when using the best power chart then lean until the last cylinder to peak was at 50 LOP.  That gave 120ktas while burning 38 lph giving us a total endurance from all tanks full (440 lts) of 11.5 hrs or no reserve range of 1390 miles.  Compared to the IO-520 we are cruising 15+ kts faster for the same fuel burn. 

ENGINE TEMPS:  I noticed you mentioned the AIM Air aircraft running hot which I thought strange since both the land plane and float plane run cooler than with IO-520s.  We tend to have cruise CHTs of about 320 across the board except for cyl #1 which hangs around 290.  All of our runways are between 200'-600' with air temps around 100 F and very high humidity.  In climb, even on hot days with several quick turn arounds our oil temps stay in the middle of the green.  The only time I have seen oil temps climb near red line was one flight that I climbed from 600' to 10500' with the cowl flaps closed just to see what it would do.  One difference may be that we are running an Airwolf wet vacuum pump and air oil separator which I would highly recommend.

SUMMARY: Last week I climbed from 200' to 9500' at 1300 kilos in 11 minutes burning 14 liters in the climb while holding Vy+ 3-5kts during the climb.  So far we are very impressed with the performance economy and smoothness of the IO-550.  As far as maintenance goes I personally find it easier to inspect the IO-550 than the IO-520 with things being more exposed but injectors and mags harder to remove and install.  Continental just came out with a TBO extension to 2200 hrs based on serial number (our land plane engine qualifies but not the Asas float plane engine) with an additional 200 hrs if you fly 40 hours per month or more.

Submitted by Rob Peterson, SIL-Cameroon on loan to Air Calvary in Gabon:

INSTALLATION / OVERVIEW:  [On a recent visit to AIM AIR (Kenya) Rob reports that…] …they (AIM AIR) have upgraded their fleet to the IO-550-N. This is a "permold" case design from Continental that puts the oil cooler in the back and the alternator in the front to meet demands of high-altitude aircraft (more-or-less).  More significantly, it is a much more balanced engine. The major design difference was making the induction tubes equal lengths for more equal airflow. It also mounts the induction air inlet closer to the inlet of the cowl allowing for more airflow. As a result, this engine is rated for 310 HP at 2700 compared to 300 HP for the IO-550-F model [on our Cessna 207 in Gabon]. The only concerns I have with the installation is the possibility of high engine temperatures with the oil cooler location (Kenya home base is at 6000' so that helps them).  I do find the engine installation somewhat more complicated overall.

EXHAUST COLLECTOR MOD:  Another modification is the tuned-exhaust system. The primary advertisement by the company is longer service life. However the reduced backpressure of the system does contribute to "better breathing" and seems to increase engine power by about 10 HP.  This is an interesting modification for [our program to consider] since the currently-installed exhaust collector’s exterior surfaces are getting rough which means they are in the later stages of life.

The engine and exhaust upgrade plus the Flint tanks (which add extra wing area) are very helpful since Nairobi is at nearly 6000' elevation and the high thermal layer makes finding smooth air difficult. They often cruise at between 11,000' - 13,000' MSL.  It would be difficult to reach 11,000' at gross weight with [our Cessna 207 in Gabon]. Today at 3700# I was climbing at about 250-300 fpm at 90 knots at full throttle and 2500 RPM.

SMOOTH RIDE MOD:  They have also modified all their aircraft with the AtlanticAero smooth ride engine mount. It reduces vibration, which helps the pilot, passengers and the aircraft (instruments). Another advantage is that it that eliminates the need to change the forward engine mounts at mid-life. N207FD front engine mounts only went 400 hours before needing to be changed. The engine was starting to rub on the cowling as a result of the sagging.

ENGINE MONTORING:  They have also upgraded their aircraft to the JPI EDM830 engine monitoring system (MAP, RPM, Fuel flow, temps, pressures, voltages). It's a nice display and works well. They have had one instance where the screen was inoperative for a time. Otherwise they have not had any problems with them. Operationally, the needles are different than the traditional Cessna needles and it's easy to confuse fuel flow & RPM.  Not only does it reduce the number of mechanic gauges but it also reduces the number of flexible fluid lines, especially in the cabin.

SUMMARY:  If we were converting the original engine, the IO-550-N would have been interesting. However, our current engine [IO-550-F) is perfectly adequate if not even slightly advantageous given the higher ambient temperatures here compared to Kenya.  It appears that our installation does run cooler than the IO-550-N in Kenya even though we are taking off from sea level.


  1. Wow 500 hours on the Asas float plane, I was involved in preparing that for service!

    1. Andy! How cool. They are enjoying the fruits of your labor! Keep up the great work brother.