The HPT 32 Crashes - An Alternate Logic

Professor Prodyut Kumar Das

The recent article in Vayu V/2012 Requiem for the HPT 32 was carefully researched and provoked thought.  The fuel divider and the Collector tank location and capacity are the prime suspects. However there appears to be some uncertainty; The article mentions that at one stage the Fuel Divider was taken off the list of suspects by the investigators and there was the case of an engine stoppage on the ground. There is a certain uncertainty.

The uncertainty could be from the following. It is true that unless the fuel supply is smartly cut off, a warm fuel injected engine will continue to “diesel” even after the ignition is shut off. This “cut off” is one of the major functions of the fuel divider. However this function “gets out of the way”, so to speak, when the throttle is opened beyond idle or near idle. If the fuel divider is the culprit all the seventy odd incidents of engine stoppage would have occurred at idle or near idle conditions. Also, if maintenance is a problem NO failures should have occurred with a new/ “not overhauled” fuel divider. Has this been indeed the case? 

Regarding the fuel pipe line being not as per FAR standards the usual requirement is that the pipeline should be able to handle one and a half times the TO fuel flow rate. For the engine in question the TO fuel flow is approximately 1.3 litres per minute or 20 ml per second corresponding to fuel flow velocity of about 0.7 metres /sec which is also not too bad. In any case the main restriction to flow would be the filter just upstream of this line and increasing the pipe diameter will not make a decisive difference. Mind you at the idle case the fuel flow would be around 200 ml per minute so both the usable header tank capacity and the pipeline would be unlikely to be a prime suspect. By my estimate, even with the usable 3.5 litre capacity quoted  the engine could chunter on for a quarter of an hour at flight idle or three minutes full chat –both times more than enough to get the aeroplane at least into level circuit if not on the ground. That too under flight conditions of no bank or turn during the descent because a bank would recharge the header! It is also to be noted that there was an engine stoppage on the ground-when the aeroplane was near as level and  feed / collector capacity problems could not have occurred. Finally it is bemusing to accept that a failure rate of 77 failures in 400,000 flight hours-that too in a system that is “on” every minute of the flight- if a single component or system is at significant fault. In my view there is clearly room for an alternate hypothesis.

Let me say my Mea Culpas right in the beginning. I had no chance to see the aeroplane or have access to the data and the hypothesis is based entirely on conjecture and my experience as an Engineer and I.C. Engine man. The starting point of the alternate construct is that considering its usage the rate of failure is very low. Could the failure be due to the fact that there are a fairly large number of random factors which almost never occur at the same time. When they occur together, however, they cause an engine “failure”. The “rare random combination” better explains the one failure every 5000 hrs. What could be these factors?

a)      The poor engine is suffering from the “Glider tug” syndrome. A high power –low air speed combination as in towing gliders have been known to cause engine failure. The HPT 32 does not tow gliders but it is an extremely “draggy” aeroplane.  If you visually compare the HPT 32 with the similarly powered SF 260 you will get the point. If you are one of those who will point out that the Italians will get “style” into concrete “tie down” blocks then look at the Finnish Vinka or even the Bravo or the Bulldog. In the HPT 32 the contours of the cowling and the canopy, the untidy undercarriage linkages and the huge fin hurt the eye. One must mention the oversize fin. The excess wetted area results in excessive parasite and induced drag leading, again, back to an overworked engine. The engine has to operate at a few notches higher throttle setting compared to other installations and yet not get enough cooling air.

b)      The cooling of the last row of cylinders in a horizontally opposed engines requires, as the Germans say, “Patience, experience and maturity”. Particularly the rearmost cylinder opposed to the direction of the propeller rotation is, cooling wise, in a severely unfriendly environment. Thus the fitment of the cooling baffles and its maintenance is of greater than usual relevance in this case.

c)      In India the cooling air itself is 20 to 25 degrees higher than ISA. This would rob it of about thirty percent of its cooling capability. Bidar is notably dry.If this is combined with the occasional less than “normal” humidity we can see problems lurking around the corner. I dare say that if the HPT 32 operated above 35° N we may not have seen this problem at all!

d)      The dust and the dirt. The metered fuel supply system takes input from the static and rams pressures. If this is not “klim bim” perfect then the mixture would lean out to the point when the engine would starve and stop. Dust would also reduce the cooling heat transfer.

So what could be happening? We have an older (somewhat dented and battered and the cowling and canopy rattles a bit in flight!?) aeroplane flying a sortie on a dry dusty day with some prolonged spirited flying at high power. The engine is hot. As the power is reduced and that aircraft is gliding back the cooling flow is reduced by the low airspeed; the heat accumulates under the cowling. May be the baffle seals are just a little aged. All add up to –in those rare occasions- leading to a local overheat, distortion and “incipient seizure” in the engine. The high oil temperature and hence reduced viscosity of the lube oil would be additional contributor in this construct.

For reasons too boring to detail here I once had a car that had 90,000 kms on the clock. It had this trick of the engine suddenly “seizing” yet when I let the old girl be for some time -this was in Daman where chilled liquid coolants for me were easily accessible!- it would restart as if there had been no quarrel. There was another case when a students’ designed racing car that would stop suddenly due to over heating. A better designed duct for the radiator cured the problem very satisfactorily. Perhaps our engine is having the same problems?

Incidentally the HPT 32’s wing and span loadings are some 40% higher than the HT2s and so the glide ratio and minimum airspeed would be that much poorer. At low altitudes the pilots have that much less a chance of a safer landing or a pancake.

If the above construct is a possible model then what is to be done?

In the immediate term:

1. To increase the routine maintenance quality. The baffling of the engine is a prime suspect and so must come for close inspection at suitable intervals. Cylinder fins and the static and pitot ports for the AF system to be checked for dirt and should be inspected for cleanliness as per flying conditions. I mention routine maintenance and preflight checks only because a recent issue of a NTSB bulletin mentions fatal crash at take off killing six people because the pilot had failed to drain the fuel tanks of accumulated water. In his previous company someone else used to do this for him.
2. The quality of fit of the cowling and the canopy joints and panels to be improved by the fitters to the extent possible to reduce drag. Older airframes to be examined for the usual dents, bumps, loose fittings with the above prognosis in mind.

 In the medium term the following studies to be made.

3. An OR study into the accidents based on the above assumption that “an unfortunate combination of circumstances” rather than major system fault is the cause of the “engine failure”.
4. Initiate the design of a neater cowling and canopy. The cowling lines of the SF 260- which incidentally has the same engine - is the work of a Past Master (Stelio Frati) and could be an inspiration. These could be retrofitted at the FTS .
5. Do a CFD study of the through flow and back flow on the oil cooler. I have seen significant improvements with some very simple “fixes” and better lubricant viscosity would be a definite palliative.
6. Do a study to find out how difficult it would be to fit a semi-retractable undercarriage as in the Yak -18 and if there would be any benefits.

The real “de luxe” solution is economically unviable but is mentioned for the completeness of the discussions. It is entirely a personal view that HAL spoilt itself by the success it had with the big fin to improve the spinning characteristics of the HJT 16 which I have seen has very reliable spinning characteristics. What worked for the Kiran was possibly tried again on the HPT 32 but the balance was lost. Optically the HPT 32’s fin is huge resulting in excessive weight and drag. Others rely extensively on strakes to generate flow across the fin and rudder in a spin. The German Grob is of course  typically Teutonic in its determined application of strakes and under fins but the Bulldog, and the Vinka not to mention the SF 260 all use strakes quite discreetly with success to achieve  desirable spinning characteristics. The gains of using strakes for good spin recovery are in weight and drag which seems to be the root problem here.

Prodyut Das

Professor

Prodyut Kumar Das is an Alumnus of St.Xaviers’ Hazaribagh, IIT Kharagpur, and IIM Kolkata. He started his career with Aircraft Design Bureau HAL and for twenty years worked and led various vehicle related Product Development Projects with leading Indian and multi National Companies.

He left Industry to join IIT Kanpur in 1993 as a Professor in the Department of Mechanical Engineering. There he won a prize of the Royal Aeronautical Society of UK for his design of a light sports aeroplane using grants given by ARDB. He also did a project study on “The design of a Light Car costing less than 1 Lakh” which was a Ministry of HRD funded project IDICM 36 and started his research on Stirling Engines in which the IN was keen.

When IIT Kanpur did not renew his 5 year tenure he returned to the  Industry as a Vice President Technical and finally retired as Advisor Aerospace in the e- Engineering Division of a Leading Indian Engineering Company.

He currently teaches Engineering in a Private Engineering College in his hometown and continues his Research as a Consultant. He has been writing on matters related to Defence Engineering since 1990s.