The HPT 32 crashes - an alternative logic.
Wednesday, 25 June 2014
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 taught 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.
Email ThisBlogThis!Share to TwitterShare to FacebookShare to Pinterest
Subscribe to: Post Comments
(Atom)
Blog Archive
§ The HPT 32 Crashes - An Alternate
Logic
Copyright © PRODYUT DAS . All rights reserved. Simple theme. Powered
by Blogger.
Comments
Post a Comment