After the purchase of a Cessna 421C in 2005 on of the first mods we performed on the aircraft was the installation of the Micro Aerodynamics VG kit. We wanted to obtain an additional margin of safety provided by the lower Vmc speed, but we also liked the additional 130 lbs of useful load. Weight carrying capacity was a primary reason for choosing the Cessna 421 to begin with, so any thing that could be done to increase weight carrying capability and increase range was something that interested us. We frequently were carrying five to six passengers in addition to me at the controls. The power settings we were using in the 421 were burning 22GPH per side and getting a bit over 200 KTAS. There are few aircraft that could do this (other than a turboprop) and , I wondered if there was something we could do to increase the range especially on the “heavy” trips. We departed at max gross weight on about 90% of our trips. Most of our trips consisted of carrying maximum passenger loads while carrying a safe amount of fuel (rather than topping off). Incidentally it was these trips that made me realize that in most cases it is best to carry enough fuel to complete a trip with a safe reserve. Less fuel means less weight. Less weight means a faster TAS. Plus an added benefit is that at lighter weights you will experience lower CHT’s because of increased airspeed and faster climb speeds for a given climb rate.
It was about this same time that I began flying a PA46-310P Malibu with a TSIO-520BE engine. I was not inclined to use LOP operations, but quickly found that this aircraft not only could run LOP very well, but really needed to in order to haul a decent load and keep engine temperatures in check. This engine and airframe were mated for the singular reason of being able to run on the lean side of peak. In the mid eighties when the Malibu was introduced few airplanes had multi probe CHT/EGT indicators. Piper did introduce the Malibu with a single TIT indicator which while rudimentary was actually quite accurate. In 1984 few people understood the science behind LOP operations, and many people refused to accept the fact that leaner mixture meant cooler temperatures. In addition to these misconceptions fuel was cheap and a few extra gallons would not hurt the pocketbook all that much. It was my operational experience in the Malibu that led me to realize that the 421C would be an excellent candidate for LOP operations.
Take a look at the TSIO-520BE (PA46-310P Malibu Engine) , TSIO-550 Engine, or GTSIO-520L&N engine and you will quickly realize that there is a fair amount of commonality especially in regards to the induction systems, and the top end (cylinders and valve train). They all have a similar style of induction system which enables these engines to have an even air/fuel distribution. This style of induction system lends itself well for LOP operations. These engines also have the cross flow cylinders with the overhead inclined valve train.
In some cases the GTSIO will run well LOP with stock Continental injectors, but I tend to think that this is not the norm. In order to get the best performance, CHT’s and smooth operations most aircraft will need to operate on a balanced set of injectors to get smooth engine operation. This will normally involve purchasing a set of balanced fuel injectors from the well know General Aviation Modification Incorporated commonly known as GAMI. My project began by ordering two sets of fuel injectors from GAMI. As many may already know the injectors which you receive are normally all one letter for example “E”. This letter refers to the orifice size of the fuel injectors. Change the size of the orifice and you change the amount of fuel flow to the cylinder. More fuel flow means the cylinder will peak later (in comparison to other cylinders), while a smaller orifice means that the cylinder will peak sooner (in comparison to other cylinders). Ideally with a well tuned engine all cylinders peak within a range of .3 GPH. The idea is that during the leaning process the first cylinder peaks and the last cylinder peaks with .3 GPH of leaning. By having the variable orifice sizes this compensates for uneven fuel/air distribution which is present in virtually every internal combustion engine.
Once the injectors are installed the process of fine tuning the injectors to an individual engine begins with a GAMI lean test. The lean test involves recording EGT temps versus Fuel Flow. I personally like to perform the lean test at about 10-12,000 ft. MSL. The reason for choosing this altitude is twofold. First this altitude is above nearly all of the traffic (in our area of East TN), and second this is low enough that a slow leaning process will not cause any high CHT problems as the 50 ROP to Peak EGT/TIT realm is traversed. I recommend a clipboard and two printed copies of GAMI’s lean test paperwork. The values could also be downloaded from any number of the Graphic Engine Monitors to an excel spreadsheet. These numbers will determine whether the each individual cylinder needs a leaner or richer injector. After reaching 10,000 ft MSL I begin the process of leaning. I normally set cruise power at 30” and 1800 RPM (engine is turning 2400 RPM). The pull begins by pulling the mixture all the way back to 25GPH. After leaning below 25GPH the temps begin to rise substantially. It is important to lean steadily, precisely, and at a pace that will prevent CHT’s from rising out of control. If you lean too rapidly it will be difficult to identify peak EGT and the engine monitor may not indicate correctly. If you lean too slowly you may end up pushing your CHT’s up over 400°F. A note here is that once you know what TIT/EGT at which your engine peaks it will be fairly consistent when flying at normal altitudes. For a given power setting you will be able to pull back to a given fuel flow and you will be very close to your LOP setting. One thing you will discover when leaning a Cessna Twin (or any twin for that matter) to lean of peak is that you must develop a precise control of the mixture in order to do it well. I found the placing my index finger on the mixture knob, and then placing my thumb in the groove where the mixture control rides provided tactile feedback to indicate when I made the slightest movement of the mixture. Those cables are quite long and the change will take a few seconds, so if you are trying to lean at .1 or .2 GPH rate you must be very PATIENT. Move the mixture a bit and wait about 15-20 seconds and see if the fuel flow changes. Continue this process until you get all cylinders to peak recording the results. It is good to take an assistant along who can assist in looking for traffic and recording data.
If you want to fly your Cessna 421 LOP, or C340, C414, or virtually any of the piston singles or twin aircraft I think the best way to do it properly and safely is to have the right engine instrumentation. I firmly believe that in order to do this it is best to have some type of Graphic Engine Monitor which indicates CHT/EGT for all cylinders along with a TIT readout. In addition to this it really helps to have a digital indication of fuel flow down to the tenth of a gallon. The digital FF readout is very important when performing the lean tests. When leaning for cruise it gives a measurable way of reducing fuel flow in a minute amount.
In order to get our Cessna 421C to purr along at LOP mixture settings we needed to do a few minor “tune up” items. First of all I recommend that anyone attempting to run any Teledyne Continental Engine LOP, that you begin by performing TCM Service Information Directive 97-3E. This SID involves setting fuel pressures along with idle mixture settings. Making sure that these setting are in line will assist in smooth LOP operation. It is also very important that there are no induction leaks. Induction leaks may be tracked by pressurizing the intake system and using soap bubbles to identify any leaks. In addition to these items it is essential that the ignition system be in top condition and that the engine timing be set properly.
Now to the results and benefits of getting the Cessna 421C running LOP. Immediately after getting the airplane configured and running well LOP I went to the AUX pages in the GNS 530 and reset the Trip Statistics, so that I could track the changes in average Ground Speed (I left the GNS 430 the same for comparison). After several months of flying we had some data to compare. Operating ROP I used 30” and 1800 RPM we truing out in the low 200 KTAS range while burning about 44-45 GPH depending on OAT and CHT. After some time spent flying LOP we discovered that we were getting about 195 KTAS, and burning 35-36 GPH. That computes to about a 20% reduction in fuel burn with a 2.5% reduction in KTAS. This translated into a much greater range. One of our trips which changed dramatically was at flight to the MYNN (Nassau, Bahamas). Instead of needing to stop at Fort Pierce and take on fuel we could go to CRG (Craig VOR), and then go straight down to MYNN. We would normally land with about 50 gallons after the flight. There were also several trips which without the range increase from operating LOP would not have been possible without a stop, but now were easily within reach. This may have contributed to the average ground speed being closer to ROP operations than expected. We were staying in cruise longer which meant less time in climb, descent, approach, and landing.
These range charts comes from my experience flying the Cessna 421C. I planned for 55 Gallons of fuel burn the first hour (both ROP&LOP). The successive hours I planned at a fuel burn of 45 GPH ROP and 36 GPH LOP. The range numbers I derived from the average GS obtained from the GNS 530. The average GS in the 430/530 is calculated from when the takeoff to landing. Our ROP average GS was 183 Knots, while the LOP GS only dropped 3 knots to 180 Knots. Based on these average ground speeds I calculated the above ranges based on the three main fuel capacities found in the 421C model. These numbers take into consideration a one hour reserve at cruise fuel flow.
The other benefits are quite typical for aircraft which are operated LOP, and included lower CHT temps, cleaner oil, better oil samples, better cylinder life, and overall better engine service. It is not all that atypical to put a set of cylinders on a GTSIO about every 600 hours. We were able to fly all the way to 1400 SMOH before we had to look at any cylinder work.
A couple of limiting factors for LOP operations which I have discovered in pressurized piston aircraft at higher altitudes may affect the maximum altitude at which LOP operations will work well. Many Twin Cessna pressurized aircraft have been modified with pressurized magnetos. This is an essential element if you want to run LOP at higher altitude. I have also found that at above about FL200 and FL210 the wastegates are closed or nearly closed all the way with cruise power settings. With the turbos providing nearly all the “boost” they are able to there is not any buffer. With no buffer any change in pitch or airspeed the MAP will vary a little bit. These engines just do not seem as smooth LOP above these altitudes. The engine is in a sort of “coffin corner”. Due to the higher altitude and more compression of the air from the turbos the induction air is hotter and therefore the CHT’s are hotter than lower altitudes. In order to keep the engine running cool enough while operating LOP it requires a leaner mixture. At the higher altitudes (above FL210) this leaner mixture results in a little engine roughness. It varies a bit from engine to engine, but certainly at the higher altitudes it is a factor.
If you have been contemplating operating your Twin Cessna on the “Lean Side” feel free to contact me for more info at (423)647-4359 or firstname.lastname@example.org or check out my website www.FlightTrainOnline.com. Watauga Flight Service provides initial and recurrent training. We provide initial & recurrent training in Cessna 335, Cessna 340, Cessna 414, Cessna 421, Cessna 425 (Conquest I). In addition to this we also provide training in Piper PA46, Malibu, Mirage, Meridian, JetProp, Matrix, King Air 90, King Air 200, Beechcraft Baron 55, Baron 58, Baron 58P, and Cessna 210 (T210, P210).