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SAFE WINGS
This issue…
ICE CRYSTAL ICING
1971 WAR: A VOLUNTEER REMEMBERS
* For Internal Circulation Only
Flight Safety Magazine of Air India, Air India Express and Alliance Air Issue 39, AUGUST 2015
F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
SAFE WINGS August Edition 39
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August Edition 39 SAFE WINGS
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F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
SAFE WINGS August Edition 39
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here are many articles on this subject in Aviation journals, however
none of them cover the events that have occurred in our airline on the
B737. High Altitude Ice Crystals have been recognised as a cause of
Engine damage and power loss that effects many different types of
airplanes. These events have occurred in conditions which pilots associate
with benign weather conditions with at most only light turbulence. The
conditions may sometimes even typically be classified as non-icing
conditions because super-cooled liquid water is not present at these
temperatures. Ice crystals normally do not adhere to cold surfaces because
the ice crystals bounce off. However the ice crystals can partially melt and
stick to relatively warm surfaces. Due to engine roll back and power loss
events, more recently on the B747-8 and 787, FAA has even issued an
Airworthiness directive (AD) 2013-24-01 on 27 Nov 2013. Boeing has also
issued a Technical Bulletin on the subject and also included procedures in
FCOM/QRH to be accomplished during suspected Ice crystal Icing. In the
B737 we have had a few suspected Ice crystal icing events which though
having some similarity to the events described by Boeing, haven't caused
engine damage or failure. The events on the 737 are described here. The
pilots reports were further authenticated by Flight Data Recording (DFDR)
data.
T
ICE CRYSTAL ICING
:- By Capt Vivek Kulkarni
August Edition 39 SAFE WINGS
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F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
In all the events that occurred in the airline to the B737, the aircraft was
between latitudes 12 North to 17 North, between Flight levels 330 to 370
and TAT was between -10 to -17. There were no Cb clouds even well below
the aircraft but the aircraft may have flown through the anvil like cloud of a
dissipated Cb. The radar returns indicated intermittent light green and there
was light turbulence. In some cases engine anti-ice was switched on except
when SAT was below -40, the engine anti ice was off.
In all the events the following was noticed:
1) The TAT increased rapidly to 0 degrees over a period of about 10
seconds.
2) Speed steadily dropped by about 10 knots. In cases where the aircraft
was at Optimum or Above optimum, due to the smaller margin between
cruise speed/Mach and Max and Min speeds, this 10 knots decrease brought
the indicated speed close to lower amber speed band.
3) The Auto-throttle increased thrust in an attempt to return to selected
The image depicts a view of a CB cloud. The aircraft has avoided the Red and amber areas on weather
radar and is passing through the relatively “safe” anvil cloud. The Avil like cloud can contain High Ice
Water Content (HIWC).
F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
SAFE WINGS August Edition 39
5 | P a g e
speed/ Mach but in spite of the Auto-throttle increasing thrust to the limit
set in the FMC (ie Cruise or MCT), it was unable to regain speed. In one
case the airplane had to be descended to regain the selected speed.
4) The thrust limit N1 had reduced by about 4 percent.
5) There was no Surge/stall or any adverse effect noticed on the engine.
Though the Industry is still not very clear how Ice crystal ice causes
adverse effects on the aircraft, we suspect the following:
Whereas super-cooled water drops is present in Vertical development
clouds above freezing levels, very minute particles of Ice Crystals are
present in high altitude clouds. Sometimes as in an anvil like cirrus cloud
which is the remnant of a spent Cb, these minute ice crystals may be
present in large quantities.
One such event described here is a firsthand account and the sequence of
events are as follows: The aircraft was at the optimum altitude ie FL 360
halfway between Mangalore and Dubai. It was night time and the weather
radar was painting intermittent green and it was obvious that the aircraft
was in cloud due to reflection from the strobe lights. There were no red or
amber returns in the vicinity even with the weather radar tilted at -3
degrees. Engine limit thrust had earlier been manually changed from
“Cruise” to” Continuous”. Due to an earlier experience the engine
parameters were noted and the Max Continuous limit displayed on the FMC
was noted as 100.5. The TAT at FL 360 was – 16 degrees and after about 5
mins in the cloud as described above, the TAT over a period of about 15
secs increased to 0 and then hovered at 0 to -1. The aircraft speed started
dropping steadily and it was noticed that the Auto throttle had increased
thrust to the N1 limit. However the N1 limit was now indicating 97.0
percent N1. Inspite of the Auto throttle maintaining 97.0 N1, the speed
continued to drop. When Auto throttle was disengaged and thrust increased
to the thrust limit noted earlier, ie 100.5, the speed decay was arrested, A
short while later the TAT started dropping and returned to its earlier value
of -16 and now the aircraft speed started increasing and thrust had to be
reduced accordingly. Auto throttle was reengaged and thereafter operations
were normal.
From the above we deduced the following. The tiny ice crystals may have
attached to the TAT probe which is heated. On contact with the heated
surface, they may have melted, which caused the TAT probes to sense a
August Edition 39 SAFE WINGS
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F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
(false) higher TAT. The TAT is also transmitted to the engine EEC. Normally
at higher temperature, due to the reduced density, the mass flow of air into
the engine is lower and hence the fuel fed into the engine needs to be lower
to maintain the air/fuel ratio. The thrust will naturally decrease. In this
case, the Auto throttle sensing the speed reducing, increases the thrust to
the N1 limit to maintain speed. However, the EEC fed with a (false) higher
temperature, reduces the N1 limit too, because it assumes that higher TAT
at the same speed means that OAT has increased.
To understand the phenomenon better, we can compare this to using
Assumed Tempe / Flex Temperature .We know that we “cheat” the EEC by
feeding it a false OAT ( Assumed temperature/Flex Temperature ).This
causes the EEC to set a lower N1 based on the “assumed” outside air
temperature. In other words we are “cheating” the engine to believe that
the temperature outside is high and it has to regulate the fuel flow
accordingly. The net effect is that less thrust is produced so that we can
achieve a Reduced take off thrust .In such cases regulations permit you to
increase thrust to maximum in an emergency such as wind-shear because
the actual OAT is much lower than what has been fed into the FMC.
A similar phenomenon probably occurs when Ice
crystals melt on the TAT probe and transmit a
“false” TAT. In suspected Ice Crystal icing if this
occurs, the thrust could be increased to the correct
MCT to prevent an unsafe condition (incursion into
the lower amber band or worse stick-shaker). You
have to anticipate though that when the TAT
indication returns to normal, the increased thrust
will increase the aircraft speed and thrust will need
to be regulated to prevent an over-speed event.
Summary
Different aircraft types have had varying events
due to Ice crystal icing. Pilots are advised to read
articles on the subject with particular attention to
events that have occured on their aircraft type,
Since the Industry is still analysing suspected Ice
Crystal Icing events, any events that may in your
opinion relate to the subject should be promptly
reported to the Flight safety department.
:
Capt V. Kulkarni is
an ex-Naval pilot. He
joined Air India in
1987 and has flown
the A310, B747
Classic, B747-400,
B777 and is presently
flying the B738 and is
also the Chief of Flight
Safety, Air India
Express.
F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
SAFE WINGS August Edition 39
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y experience of the 1971 war began a few days before the outbreak
of hostilities. I was at that time doing ground duty tenure at CAC
Allahabad. The buzz in the offices and corridors of our building was
palpable. There was an air of detached sense of expectancy.
Meanwhile in the squadrons the imminent certainty of war meant that all
preparations were well and truly in place in a measured way. Unlike the
pre-1965 war scenario induction of pilots on ground duties to bolster the
pilot strength in Squadrons was not a critical need. To me however the
comfort zone of a desk job away from the reality of action was
disconcerting. Call it payback time or an expression of confidence in oneself
built on the back of my 1965 operational experience I volunteered for action
and was quickly assigned to No.35 Sqn. then based in Poona.
Poona was the launch pad for targets in southern Pakistan. The limited
range and reach of Canberra aircraft at low level meant a refuelling halt at
Jamnagar on our way out to targets. It proved to be a welcome break for us
to meet with Gp.Capt.Pete Wilson Vr.C. Stn.Cdr. Jamnagar, a veteran of
Canberra operations and my mentor in more ways than one.
Of the 6 night missions that I undertook from Poona, 2 are worth
mentioning here; one for a lucky escape and the other for a side show
narrative to the leaping flames of Karachi on a night raid.
M
1971 War: A Volunteer Remembers
By :- Capt K.S.Chandrasekar
August Edition 39 SAFE WINGS
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F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
And so it was on 8th Dec.1971. after a reassuring tarmac chat with Groupie
Wilson over hot tea on a cold and wintery night on our way to Drigh Rd., a
satellite PAF base close to Karachi. No worries and mission successful, I was
down to deck level over the dark waters of the Arabian Sea heading south
away from the prowling eyes of Pak radar , eased my body, silently ebbing
away the adrenaline. Close to Jamnagar I had just initiated my climb to
Poona when the silence was broken with the calm voice of Groupie Wilson
asking me to get down fast. No questions asked, I realised I was not the
only one in the night sky at that point. Instinctively I went into a spiral dive,
throttles back, speed brakes out to hit the deck back to the safety of the
black and glistening surface of the waters whizzing past below. After what
seemed an eternity at low level and reassured with an all clear from
Groupie Wilson, I made my way up again back to Poona with near dry
tanks. Back again on 9th. Dec., another mission and another tarmac chat
with Groupie brought home the reality of the previous night's close call. The
Pak chase plane ( had to be a F-102 for it to gain on me so rapidly ) was
picked up by our radar in time to wake me to action. The rest is safe inside
me since then. Lucky are the ones that live to tell a tale another day.
The side show of the night on 15th. Dec. 1971 (the last day of the war).
This was a night raid on the Karachi harbour installations. As leader of a 6
aircraft raid, planning and briefing carried out, we were off in a stream from
Jamnagar on a circuitous route via Bhuj, approaching the target from the
north. Even far out at low level it became clear that navigation that night
F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
SAFE WINGS August Edition 39
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was going to be easy. We had a homing beacon in the form of a gigantic
inferno lighting up the night sky from miles away. Our Hunters and Naval
boats beat us to the job earlier and made our job easy by lighting up the
target. Closing in, the leaping flames were riveting and yet distracting. As
we made the final bombing run, the effect of our bombs hitting the target
reignited another burst of shooting flames sky high. A spectacle to carry
away with you forever as I sped away with blinking eyes and a nodding
head in disbelief at the sights of the night.
Capt Chandrasekar joined
Air India in 1976. Prior to
Air India he served in the
Indian Air Force and retired
as a Wg.Cdr. He was
awarded a Vayu Sena Medal
(Gallantry). During his
career in Air India he has
flown B707, A300 and B747
and has been a Training
Captian throughout. He is
currently a Simulator
Instructor with Air India
Express on B737.
August Edition 39 SAFE WINGS
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F l i g h t S a f e t y M a g a z i n e o f A i r I n d i a , A i r I n d i a E x p r e s s a n d A l l i a n c e A i r
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PROMISING A SAFER SKY, AIR INDIA, AIR INDIA EXPRESS & ALLIANCE AIR
We give utmost importance to your valuable comments and feedback. Please do mail us at
Editorial: -Capt. V. Kulkarni, Bhavish B S, Designed by Bhavish BS
