Whilst
collecting material for a feature on prospective Indian Civil aircraft I came
across the DGCA report “The Final report
on the investigation on the accident to the NAL Saras PT2 aircraft VT-XRM at
Sheshagirihalli near Bidadi’ Karnataka on 6th March 2009” report
dated 6 December 2009. The full report
of 75 pages is available at dgca.nic.in/accident reports. I understand that
there are no other reports available to the public.
I
am uneasy with the findings of the report and its recommendations because of
the following:
i)
The
report does not explain the type of wreckage or the wreckage “foot print”.
In
addition the report does not seem to have investigated the following:
ii)
The
configuration of the aircraft ( rear engine T tail turboprop)
iii)
“deep stall”
iv)
flat spin”
v)
The
possibility of the CG position of the aircraft having travelled beyond the rear
CG limit.
Without
having investigated and ruled out the above possibilities the report may have
drawn erroneous conclusions and corrections.
If my prognosis is
correct the aircraft may crash again if tested under the suggested corrected
test parameters.
Unfortunately
the report does not contain the data needed for me to check my own
apprehensions. Since the Saras test flying has resumed I am taking the risk of
publishing my views for a wider discussion.
Introductory
Glossary:
The rear engine
twin turboprop configuration:
The
engine, nacelle, propeller and mountings of this class of aircraft- twin engine
light commuter- is around 15% of the mass of the aircraft. If such a mass is
located at the rear, as in the Saras, the CG moves backwards by a considerable
account.
To
“balance” the aircraft for stability the wing has to be moved back. This
reduces the tail moment arm significantly and it is normal for this
configuration to have either reduced yaw and pitch stability or have larger
keel (fin) and stabilizer areas so as to maintain “tail volumes”. The lateral
stability is worsened by the fact that there is a larger “Side area” ahead of
the CG further reducing the yaw and pitch stabilities.
The
location of the undercarriage is more sensitive in a rear engine layout. It can
affect the stabilizer sizing (by about 30% in one case). However there is no
evidence that this aspect played any immediate role in this accident and is
therefore not being further discussed here.
The spin
An
aircraft if yawed at near stall speeds it will inevitably flick into a spin. It
happens as follows:
The
aircraft is flying straight and level at near the stall speed. If yawed, say as
in this case, to the left the left (port) wing’s airspeed falls and it
generates less lift. This causes the wing to dip which increases the angle of
attack of this left (port) wing. It is now well and truly stalled. The wing is
past the inflexion of the Lift /Drag curve and so it generates less lift and
more drag. Exactly the opposite happens to the starboard (right) wing. The entire
wing is now subjected to a rolling couple (because of the different lifts) and
a yawing couple because of the different drags. The aircraft becomes
uncontrollable because of the low airspeed which makes the control surfaces
forces much less than in normal flight. Recovery happens by diving the
aircraft- to get the entire wing “flying” again- and correcting the yaw. A spin
is a frightening situation to be in and requires preparation and mental
“psyching”. Due to the lower moment arms a rear engine aircraft takes longer to
recover from a spin.
Deep stall
At
stalling speed an aircraft is at around an AOA of 15-18 degrees. This may
increase as the aircraft begins to sink due to loss of lift. In a rear engine
aircraft with a T tail the disturbed airflow past the engine nacelles “hits”
the stabilizer and reduces its effectiveness partially or completely. The pilot
is unable to pitch the aircraft nose down to recover from the stall. The BAC
111 prototype accident in 1963 brought this phenomenon to public attention. The
Deep stall is peculiar to the rear engine layout.
The alternative
construct of the accident
The
starting point of this hypothesis is the examination of the impact point and
the description of the wreckage:
1.
The
ground is hard and stony. The wreckage should have slid along and formed the
usual “wreckage trail”. This has not happened.
2.
An
aircraft has fallen out of control from a considerable height (about 7000’).
The scatter would have been like that of a bomb explosion with an
elliptic/asymmetric scatter of debris.
These
are not reported.
3.
It
is reported that all the components are within a 20 mts. radius of the wreckage
i.e. hardly greater than the wing span of the aircraft. This was caused by the
fuel tanks exploding and not due to kinetic energy As reported rightly, this
indicates there was little or no forward speed.
This type of the
wreckage “footprint” happens when the aircraft has crashed whilst in a flat
spin or deep stall or if the aircraft CG has travelled beyond the rear limit.
How did the spin
happen?
The
aircraft as per report was at around 132 knots at the beginning of left engine
shutdown. The speed decayed further due to additional rudder trim drag and due
to lower power from one engine. At the beginning of the engine restart point
approx -1.32 minutes time before the impact the speed was around 120 kts. At
that altitude and at the probable then weight of the aircraft the stalling
speed was around 115 kts.
When
the propeller was accidentally unfeathered, the asymmetric drag pulled the
speed down close or below the stalling speed and also yawed the aircraft. The
spin then became inevitable.
The
spin then developed into a flat spin
which was probably mistaken by the crew as a (momentary) recovery. If the
aircraft was in a normal spin it would have impacted “nose down”. This is not
indicated by the wreckage. Recovery from a flat spin is impossible. The crew
did not have a chance.
Deep stall
According
to the report the aircraft was losing height at 10,000 fpm. This works out to
around 95 kts. The IAS of the aircraft had (to be noted) further decayed to
around 110 kts. The relative wind was in the order of 60 degrees. Even if nose
down pitch angle is considered (the report is not clear on this) the aircraft
was definitely stalled and the stabilizer was in the disturbed air of the
nacelles and therefore ineffective.
Rearward shift of
the CG.
The pilots
repeatedly complain about the excessive drag. The Saras aircraft is not per se a
“draggy” layout (note 1). From the TO weight given the aircraft was not
ballasted for the passengers. In a rear engine layout this results in the CG
moving back. It is quite likely that the CG had started at a very rear ward
position and as the fuel was consumed it
moved further back beyond the permissible limit. This made the aircraft
uncontrollable. An aircraft out of trim- like a car being driven with the
parking brake “on” will feel “draggy “and be ready to depart control. This
condition can demonstrated easily and simply.
Other factors.
1.
The
pilots had over 2000 hours experience but
less than 300 hours on turboprops. They were likely from the fighter
stream. Test piloting is a precision engineering job.
2.
The
aircraft had a three lever engine/prop control unlike the usual two lever
system. This particular test required instant and instinctive reactions as well
as utter familiarity with turboshaft engines-
preferably not even turboprop. The Crew( apart from the Flight Engineer) were
not suited for this task ( Note 2)
Conclusions:
The
aircraft crashed due to problems associated with the layout, deep stall and a
too rear position of the CG or a combination of all three.
If
the test is repeated at the same speed but even
with 15000’ AMSL altitude recovery from uncontrollability is not certain. The
low speed close to the stall was definitely a factor.
The verdict of
pilot error is to be reviewed and downgraded.
Recommendations
1.
The
spin recovery and the deep stall characteristics of the Saras needs further study
and revalidation
2.
It
is presumed that prototypes were under 24X7 CCTV surveillance and monitoring. Prototype
security on the ground is mandatory and standard assumption in accident
investigations. IF not unauthorized tampering and or sabotage cannot be definitely
ruled out.
3.
The
behavior of the two pilots prior to the start of the tests, the wrong reporting
of heights etc and the decision to press on despite misgivings of all three
needs further investigations. It is presumed that the post mortem included checks
regarding absence of harmful medication.
4.
The
verdict of pilot error needs review. They were not briefed about the
possibility of the aircraft spinning. Any flight test plan should be subjected
to “Failure Mode Analysis” so that pitfalls are not overlooked. The failure to
anticipate the possibility of proximity to stall speeds at 10,000 feet is
worrisome.
5.
The
report should include, in tabular form, the aircraft attitude parameters, the
altitude and the control deflections, particularly for event after -1.36min.
6.
It
should be mandatory that any crew member can order abort of a sortie and return
to base for debrief and check. It is cheaper and faster than accident
investigations and building another prototype.
7.
A
sortie cannot depart until all instruments and sensors are tested for
uniformity of readings.
If
the root cause of the accident is not correctly identified then these men have
died in vain.
Note 1.
There
is no reason to surmise that the drag reported by the pilots is inherent in the
design. Some views have been heard that the fuselage diameter is too large for
the aircraft and that the wing aspect ratio is too low.
The
fuselage L/D ratio is around 7.5:1 and is suitable for this class of aircraft
and speeds.
The
aspect ratio of about 8:1 is standard practice for Russian designs who use this
even in their long range bombers. Interestingly the Americans use around 11:1
and suffer from structural fatigue problems. The C5A Galaxy had its entire wing
replaced in most airframes and the B 52 has had so many “Band Aid” patches it
is claimed that the airframe carries its own weight in these reinforcement
patches. Since induced drag is a reciprocal of AR the Russian approach is
technically more correct. This is a personal opinion.
Note 2
Flying
safety requires quick reactions to the point of being instinctive. Wg. Cdr. Guy
P. Gibson V.C. the legendary bomber
pilot of the RAF died in a crash of the DH Mosquito aircraft to which he had
recently converted and was out of practice. It is surmised that when one of the
fuel tanks ran dry and the engines cut delay in switching tanks because of his
being out of practice may have caused the fatal crash.
