(Guy Leitch Flight Test) – As Avgas prices head for R40/ litre the need for an alternative to Avgas powered planes becomes ever more pressing. The good news is that, for the past 16 years, Jet-A diesel piston engine technology has been quietly maturing. It has now become a well-developed aero engine that is a serious contender to compete in the piston market.
MOST C172S ARE POWERED by the clunky and antiquated Avgas burning Lycoming 0-320, or the even older six-cylinder Continental 0-300. The question this flight test asks is whether the Jet-A burning 155hp Continental CD-100 powered Cessna 172 is a real alternative.
The threat to Avgas supplies has been around for twenty years. A major step towards freeing the General Aviation industry from Avgas happened in 2006 when an enlightened SACAA allowed South African flight schools to train on non-type certified aircraft. However, Mogas is prone to vaporisation and many piston singles, particularly low winged types, proved unsuitable for a Mogas STC.
Despite a much hoped for revolution in training aircraft and a reduction in costs as schools embraced Mogas-burning trainers such as the Sling 2, (which will run on car petrol; so-called Mogas), there has also been a huge shift back to legacy Cessna C172s. The demand has been so great that nowadays you can hardly find a clapped-out 60-year-old C172 for less than R1 million.
At the time SA Flyer noted that; “Non-ACSA airfields, managed by property speculators rather than intrinsic service providers, have shown little enthusiasm in encouraging aviation by installing Mogas pumping facilities. However, the provision for supplying Jet A1, the motive fuel upon which our entire global transport infrastructure exists is an entirely different matter. Big jets mean big hangars and big maintenance facilities and big volumes of fuel = big profits.
‘a serious contender in the piston market’
With problems with both the availability and quality of Mogas, running a piston engine on Jet A1 is a logical alternative, but it was a solution which was surprisingly slow to develop.
In the early 2000s, German engine builder Thielert emerged as a major force in the production of Jet-A diesels when they developed their Thielert 135 for the C172 and smaller Piper Cherokees.
The first Thielert-engined 172 arrived in South Africa as far back as 2006. That 172, ZS-PRZ, was a 1979 172N model which was eventually acquired by local aircraft owner Milenko Krsmanovic.
Fast forward 16 years and Milenko was proudly displaying not one but two Jet-A diesel powered C172s at the Aero Expo at Wonderboom. He was keen to show us what the improved 155hp Continental CD-100 Jet-A diesel C172 could do.
The STC Conversion
The first C172s converted in South Africa had the 135hp Centurion 1.7 Thielert firewall-forward modification. This includes changes to the fuel tanks, a new MT three-blade composite propeller and modifications to the instrument panel, throttle quadrant and fuel selector.
Unfortunately the small Thielert 1.7 hit early growth pains. Most notably, the prop speed reduction gearbox proved an expensive weak point and was mandated for a change every 300 hours. There were also problems with cooling – particularly in the hot and high highveld flight school operational environment.
The Thielert aircraft Engines GmbH company collapsed in 2008 and AVIC International eventually acquired the assets of Thielert in July 2013. AVIC International had acquired Continental in 2011 and so both companies became sister companies. But happily, the development of the Jet-A diesels continued in the background and the CD (for Continental Diesel) range of engines are now well-proven, reliable – and finally, as we shall see, a compelling financial proposition – especially for high utilisation applications such as flight schools.
Compared to the traditional ‘classic’ engines, there are some idiosyncrasies in the operation of FADEC Jet-A diesel aircraft engines. Notably,a problem that causes the FADEC to record an exceedance disables the engine after shutdown. These may be slight, as Milenko discovered after one flight. A heavy sideslip led to a momentary break in fuel flow to the engine management system – caused by yawing unporting the fuel-feed pipe and a small air bubble entering the fuel system. The FADEC denied an engine start on the following flight. Fixing it required connecting a laptop to the unit’s data port and emailing the data file back to the factory. The company’s engineers diagnosed the exceedance and by return email sent back a re-set code.
The key cost of operating the CD-100 JetA- 1 diesel is maintenance and in particular, the engine overhaul. Since Continental took over many improvements have been introduced to the engines The CD-135 and 155 engines’ life limit has been extended to 2,100 Hours. Once the engine’s life limit has been reached, it is not overhauled but replaced with a new engine.
‘Running a piston engine on JET A1 is a logical alternative’
A Life Extension Program (LEP) has set key components such as the gearbox to a 1,200 hour overhaul or replacement. A key development has been the introduction of a dual mass clutch to the new gearbox. To reduce strain of engine start, a start monitoring loom has been added.
Continental now have more than 2,000 Jet-A diesel engines in operation, reporting upwards of 9 million hours of operations. Their website says that all Continental Jet-A diesels are certified by FAA, EASA and a further 78 countries. The engines are particularly favoured by flight schools and specified by major OEMs, including Tecnam, Cessna, Diamond and Piper.
Both the CD-135 and the CD-155 engines are available as a complete retrofit kit. These kits include the engine, the engine mount, the propeller, dual redundant FADECs, all wiring harnesses, electronics, instrument panel gauges, accessories and parts required to install a Jet-A engine.
The Modern CD-155
To find out just how far the CD range of Jet-A diesels has come, we checked out a Cessna C172, ZS-SFT, fitted with a 155hp version of the Continental CD Jet-A diesel. Although most Avgas burning C172s now use either a 160hp or 180 hp Lycoming four pot banger, I expected the performance of the low powered Jet-A diesel version to knock the avgas burner out the ring.
The Jet-A diesel has three clear advantages – it has a constant speed prop, is turbocharged and has the bulldozer torque of a diesel engine further increased by having been geared. Yet the only evident external difference to a standard C172 is the three-bladed MT propellor. A closer look reveals a large silencer – which keeps flight school neighbours happy, and an additional small duct on the right side of the cowl for engine induction air.
Flying the CD-155 Cessna 172
There are almost no external clues to the serious engineering work beneath the cowl. Unlike other attempts to drop in a Jet-A diesel engine such as the SMA engine into the Cessna 182, the CD-100 series Continental has managed to retain the 172’s nose profile without substantial re-shaping, bulges and air ducts.
‘This may dismay traditionalist flight instructors’
The cockpit is little changed except for one obvious difference – a single throttle lever that replaces the traditional carb heat, mixture and throttle verniers – and for the few ‘classic’ constant speed C172s, the prop pitch control.
A closer look shows important additions to the lower panel, including new switches for ignition, battery and fuel pump. Above these are a pair of LED gauges providing engine speed data, temperature and pressure information and a Full Authority Digital Engine Control (FADEC) master panel.
The Continental CD’s single FADEC with dual circuit boards is the heart of the engine’s control system. Like common rail diesel cars, this makes the plane entirely dependent on electrical power to keep the engine running. A failure of the alternator therefore leads to a standstill of the engine if the battery is discharged.
The main battery is the first redundancy in case of alternator failure and ensures the continued operation of the engine for up to 2 hours, depending on other electrical consumers. As an additional redundancy, a FADEC back-up battery is installed to ensure engine operation for a further 30 minutes – time wisely spent looking for somewhere to land.
There is a separate master switch which allows the pilot to quickly shed load in the event of an electrical problem in the air. A great FADEC advantage is its ability to control fuel flow and rpm, the unit taking care of propeller pitch.
Being an ignition-compression engine (ICE), fuel needs to be fed into the system under pressure rather than via the Cessna usual high wing gravity-feed – hence the auxiliary fuel pump, which is used for takeoff and landing.
For the engine start; with the master, battery and alternator switches on, a button is pressed on the master control panel to switch on the pre-heaters. As soon as the orange pre-heat light goes out, the starter key can be turned. The engine fires up smoothly, without the gearbox clatter of a Rotax. Like a modern car, there is remarkably little fuss with the start process and the engine quickly settles down to a smooth hum at idle.
Pressing the ‘CED’ button tests the two engine gauges, sending a ripple of lights across the right-hand gauge to establish the health of the various temperature and pressure probes. Special attention is given to the ammeter to check for a positive charge. Then it’s back to normal Cessna 172 SOPs flight controls, flaps, instruments, avionics and other pre-taxi items.
Thanks to its liquid cooling it only takes a couple of minutes for the engine to warm up prior to completing the unique pre-takeoff vital actions. With the oil temperature at +50C and the coolant at +60C, the throttle is set at idle and the FADEC test button is depressed and held. The engine runs through its run-up checks automatically.
It takes a while to get used to the uncommanded engine power increase and then decrease twice and the propeller being cycled twice as the FADEC tests both systems. This may dismay traditionalist flight instructors, but others will find it a happy glimpse into the future of powerplant convenience and a welcome move away from ancient engine technology.
Cessna 172 pilots don’t expect head snapping acceleration and in the Continental 172’s case, 155 horsepower might be interpreted as a step backwards in performance expectations. However, the Continental CD-100 engine is
turbocharged has great torque and a constant speed prop, so even the 135hp version produces more static thrust at 2300rpm than a 160hp Lycoming from a standing start.
Once airborne, thanks to its constant speed prop, even at sea level the 155hp Continental 172 has a vastly improved climb rate over a normally aspirated 160hp 172. By 6000 feet classic 172’s Lycoming O-320 is already gasping and producing just 128hp, whilst the Jet-A diesel produces its 155hp all the way to 9000 ft. This means far better runway and climb performance at Highveld circuit altitudes.
Climbing at 75 KIAS two-up with half fuel you can expect a healthy 700 fpm, which is great for a humble C172. And the turbo and constant speed prop benefits cruise speed as well, with the Jet-A diesel 172 pulling away in the cruise at the 8,000 – 10,000 feet most inland cross-country flights are flown at.
‘2000 JET-A diesel engines with more than nine million hours’
In cruise, the Jet-A diesel 172 is surprisingly quiet and smooth. Being a turbocharged aircraft, speed improves significantly with height. Milenko reports that 110 -115 knot cruise speeds are realistic at FL75 and above.
Range is a big plus ZS-SFT has 52 US gallon tanks. Burning just 5.5 gallons an hour gives a bladder busting 8.5 hours endurance at 100 KTAS – for a 900 nm range. Not having the hassle of having to refuel at every stop on a long cross country is always a pleasure.
In all other respects this is a standard Cessna C172 with all the stability, good manners and viceless handling that has made the C172 the world’s best-selling aircraft.
C172s are great teachers. They are easy to land with a classic flare and nose-high holdoff attitude. There is perceptible nose heaviness with the Jet-A diesel version which seems to require somewhat more aft-stick during the holdoff – but the aircraft feels otherwise identical to a standard model during all phases of flight. As an aside, for flight schools, this nose heaviness is good preparation for an upgrade to the notoriously nose heavy C182 and C206.
‘The engine is inclined more than 45 degrees‘
The full firewall-forward conversion costs U$70,000. It may seem a lot, and risks over capitalising an old C172 but, in a flight school it soon pays off. Milenko is always happy to show with some quick mental arithmetic
The TBO on the Continental Jet-A diesel is 2,100 hours. If it burns 20 litres/hour of Jet-A which costs R25 / litre it will have used R1.05m worth of fuel in that 2,100 hours. In comparison, a classic C172 with an 0-320 with a 2,000 hour TBO burns around 30 litres/hour of Avgas. At an average cost of R35/ litre = R2.1m which is exactly double the fuel cost of the Jet-A diesel – and is more than enough to pay for a brand-new Jet-A diesel engine at TBO. So you get a brand-new engine, and not some questionably overhauled and reassembled 2000 hour parts
At today’s exchange rate, the fuel saving alone will probably pay for the conversion over the 2,100
hour TBO – less than three years’ flying for a busy flight school 172.
For private owners looking for a cross country machine, at a high 80-percent cruise power, the 155hp Continental burns around 24 litres, or seven US gallons, an hour, which is some 25-percent less than a standard 172. With the cost of jet fuel typically 33-percent less than avgas, the Continental 172 effectively slashes two thirds off the fuel cost.
The key return on investment comes from the lower fuel burn and the cheaper fuel – and of course Jet-A is far more readily available, especially in remote parts of Africa, than Avgas.
All in all, there is a lot to recommend the Continental CD-100 diesel powered Cessna 172.
Safair Campus, Northern Perimeter Rd, Bonaero Park, Kempton Park 1619, Johannesburg, South Africa. Phone +27 10 003 3747 Email email@example.com
Specifications and Performance
|Continental CD-100 Cessna C172
|27 ft 2 in (8.28 m)
|36 ft 1 in (11.00 m)
|8 ft 11 in
|174 sq ft
|Modified NACA 2412
|52 US gallons
|1 × Continental CD-100, 155 hp
|3-blade composite constant speed
|47 KIAS (power off, flaps down)
|750 nm @75% @ FL100
|Rate of climb
|14.1 lb/sq ft