TREATING ENGINES BADLY
Phil Philips, a leading light in the world of gliding and a very experienced engineer, has something to say about shock cooling. This article was originally published in the GASCo Flight Safety magazine and is very useful reading for all light aircraft pilots.
The most common engine for GA aircraft remains the Lycoming range. That they were designed more than 60 years ago and have hardly changed since, shows they were a good design for the time.
UK gliding typically uses Pawnees, Super Cubs, and Robin DR400s to aero tow launch gliders. These use Lycoming engines between 150 and 260HP.
A normal flight profile is to tow the glider at between 50 to 70 knots to 2000 ft using full power and then to get back on the ground with the minimum delay. All too often, minimum delay meant closing the throttle and descending at high airspeed which was bad news for the poor old engine. Just think, in a life of 2000 hours which we now invariably achieve, this equates to 12000 full power climbs and descents with no time at a steady cruise power setting harsh treatment for any engine.
In the four years between 1987 and 1991 Lasham, probably the largest gliding club in the world, operated 6 tugs and did over 60,000 aero tows. The Club bought two new engines in most years. However, they also had to replace 28 cracked cylinders in this 4 year period. Most cylinders cracked between the valve seats and into the exhaust port or to the plug-hole, but 2 actually blew the head right off the engine. The symptoms were obvious, a significant loss of power, rough running, and worried pilots.
Economy demanded that we rewelded or bought reconditioned cylinders; this was still expensive and had flight safety implications. Initially it was all rear cylinders that were cracking and we alerted our pilots to monitor the CHT carefully and speed up if it neared the redline. The following summer showed this was not the answer. We cracked another 10 cylinders, 6 rear, 4 front. By the end of the 4 years this became 18 rear cylinders and 10 front. Clearly our method of operation was inducing severe thermal stresses.
To study the problem Lasham fitted an engine with approved flight test engine instrumentation. This immediately showed the damping and lag in normal CHT gauges which made them unsuitable for cooling trials. Lead by David Oliver, the Tug Master, we made a lot of flights recording CHT v height and time for each cylinder. We confirmed that rear cylinders got the hottest, usually by ISO C, and found that from full throttle the rate of cooling of hot cylinders was dependent on quite small reductions in power. Just 50 RPM reduction starts cylinder cooling without an increase in airspeed. Increasing the airspeed at the same time gave a cooling rate faster than the Lycoming recommended maximum of 28° C a minute. We were in a cleft stick! We needed to descend but we could barely throttle back or we might crack another cylinder.
Fortunately the better instrumentation enabled us to develop a practical technique for throttling back a hot engine. The critical time is the first few seconds, too much rpm reduction or any increase in airspeed causes over cooling. Too little rpm reduction simply delays the start of cooling and prevents descending.
We changed the operational technique so that:
After the glider release, rpm was slowly reduced over 10 seconds, from 2550 to 2400, height was maintained or increased a bit so there was no significant increase in airspeed. During the next 10 seconds, rpm was reduced to 2300 and a speed was gradually increased in a descent. After this the 2300 was maintained and the airspeed steadily increased to 110 knots. After a minute the CHT will be below 200 degrees, out of the danger zone, and appropriate power settings for the rejoin could be used.
We also bought only NEW engines and NEW cylinders. The engines had to have solid crankshafts to avoid the dead band. What did this achieve? Cheaper engines and improved safety. We now rarely have to change a cylinder and sometimes get up to 2600 hours out of an engine. We also showed that pilots, once they are using the right techniques, can make a huge difference to engine life and can keep our Lycoming engines going perhaps for another 60 years.
In your GA flying think about thermal shock and never go from full power to idling especially combined with an increase in speed - your cylinders are at considerable risk! Think 'engine comfort' for safer flying.
From GASCO Flight Safety
