I write this in the wake of two recent tragedies involving conventional rotorcraft where amateur footage of the helicopter spinning out of control has stirred the collective conscience of aviators worldwide.
There are few critical emergencies in a helicopter that can overwhelm even the best pilots. Complete loss of directional control is one of them. At low speeds when aerodynamic surfaces are ineffective and torque reaction has no counter, loss of tail rotor can quickly spiral the helicopter out of control. Expect no graceful degradation here.
Turboshaft Engines Reliability
As per a 2016 IHST-CIS study on ‘Helicopter Accidents – Statistics, Trends & Causes’ (see it here), in commercial air travel and aerial work, 22% of helicopter accidents by classification ‘immediate cause’ have been attributed to ‘system failure’ (2005-2015 data). Improvements in aeroengine design has made them one of the most fail-safe components installed on the helicopter. Yet, the IHST-CIS study indicates that a whopping 75.9% of ‘system failures’ have been attributed to engine failure.
Remember, on a single-engine helicopter the only way to go after you lose that engine is down (Performance Class 3).
Category A Certification
Every helicopter certified today falls either into Category A or Category B.
Cat A certified helicopters can be operated in Performance Class 1, 2 or 3 but Cat B helicopters may be operated only under Performance Class 3. Depending on where you want to put your money, what kind of operations (under what regulations) you undertake, the machine you fly will fall into one of these categories (read an interesting article on the subject by Frank Lombardi here).
Operations over ‘hostile terrain’ such as offshore are mostly undertaken under Performance Class 1 (thus ruling out single-engine helicopters). Category A certification has almost become an industry norm for twin-engine helicopters. On CAT A certified helicopters, even if you lose a critical power unit during takeoff or landing (when operated as per prescribed procedures), a safe ‘flyaway’ or landing can be achieved. Since such performance may not be achievable across the flight envelope, flight profiles, Cat A weight altitude temperature (WAT) limits and performance charts are prescribed by manufacturers in the Rotorcraft Flight Manual (RFM). Deviation from these ‘profiles’ and WAT limits do not guarantee a safe flyaway or landing.
But what about that thingy at the back?
But what about loss of tail rotor(TR), TR drive or yaw control on helicopters meant to give you safe OEI flyaway or rejected takeoff? Somebody please cue the soulful music 🙁
Looking deeper into the percentage of accidents caused by ‘system failures’ reveals that of the balance 24.1%, the lion’s share of system failures – almost 14% – pertain to failure of the TR or its ancillaries. After eighty years of being around, the TR still remains a single-point vulnerability in conventional helicopters even as manufacturers develop better systems and procedures for CAT A performance.
First AW169 Fatal Accident
The 27th Oct 2018 crash of an AW 169 (G-VSKP) while executing a high-performance, near-vertical takeoff from King Power Stadium, Leicester is the most recent, tragic example.The UK AAIB investigating the crash of G-VSKP has released two Safety Bulletins post the fatal accident. The failure sequence as per UK AAIB’s second Special Bulletin S2/2018 states that:
“The evidence gathered to date shows that the loss of control of the helicopter resulted from the tail rotor actuator control shaft becoming disconnected from the actuator lever mechanism. Disconnection of the control shaft from the lever prevented the feedback mechanism for the tail rotor actuator from operating and the tail rotor actuator from responding to yaw control inputs. Loss of the feedback mechanism rendered the yaw stops ineffective, allowing the tail rotor actuator to continue changing the pitch of the tail rotor blades until they reached the physical limit of their travel. This resulted in an uncontrollable right yaw.”
An amateur video of the accident captured by a spectator can be viewed here (disturbing video, viewer discretion is advised).
Another, very recent, footage of an AW139 helicopter that crashed on 29 Dec 18, reportedly after clipping a zipline in UAE, shows a rapidly spinning helicopter plummeting to the ground and exploding into flames (disturbing video, viewer discretion is advised).
The helpless situation is there for everyone to see.
Tail Rotor Trim Thrust or Rudder Trim
For every power setting of main rotor, a specific anti-torque thrust is required to maintain direction (in nil wind). This setting, called tail rotor trim thrust or rudder trim, will dictate the helicopter’s reaction if directional control were to be lost, partially or fully. Relative wind velocity, environmental factors, design of stabilizing surfaces and their interaction with downwash and vortices shed by the main rotor may either add or subtract from the trim thrust.
The situation can be much worse during takeoff or landing, especially high performance CAT A vertical & back-up takeoffs that render stabilizing surfaces ineffective or even destabilizing (the helicopter, like an arrow, is designed for forward flight). Low speeds (below 30 knots) present aerodynamic discontinuities that are hard to quantify or define under all conditions – one of the reasons all helicopters undergo additional flight testing in low speed stability and control margins. The result of such testing is the ‘critical wind azimuth’, crosswind, & tailwind limitations (and Area A/Area B defined in some RFMs).
Tail Rotor Troubles
Tail rotor troubles can range from handling qualities degradation in adverse wind angles, to loss of tail rotor effectiveness (LTE), partial TR control (fixed pitch failures), TR control binding and complete loss of TR control due drive shaft failure, tail strike (including TR wire strike), separation of tail gear box (TGB), tailboom, etc. Every helicopter pilot must have seen or heard of at least one accident in their circles. I recall at least seven, among people i have known, few of them fatal.
Emerging eVTOL venture capitalists and folks like me who have done time on unconventional helicopters like Kamov-28 ‘Helix’ (coaxial), K-Max (intermeshing), Chinook (tandem), etc. will regard such failures with ‘falcon’ eyes (‘Falcons’ nest the coaxial KM-31 AEW helicopters of the Indian Navy).
Complete Loss of Directional Control
Alouette IIIB on Final Approach. In 2005, a light, single-engine helicopter on the last leg of a triangular non-stop cross country training sortie likely experienced unusual behaviour from the tail rotor (estimated, in the absence of FDR/CVR and no survivors). The instructor sized up the situation adequately enough to make a force landing, stop rotors and send a crew member to take a walkaround and check if all was okay (untrained eyewitness account). Maybe nothing was found amiss because he decided to pick up and continue to base, about 12 miles away. A fatal mistake as it turned out. Minutes later, the helicopter spun out of control and crashed short of the runway with loss of all three lives onboard. It was estimated that a debonded TR blade had excited divergent vibrations that led to complete loss of TR drive within minutes. A ‘mushroom rivet’ on the HAL-manufactured TR blade was called into question.
Dhruv (ALH) in Cruise Flight. 2005 again. A twin-engine Dhruv (Advanced Light Helicopter) in cruise flight with a HAL test pilot (TP) on controls had a more miraculous outcome. Experiencing sudden vibrations from the tail section, the TP gauged the situation quickly, feared the worst and alerted his copilot about intentions and immediate actions. Within seconds, divergent vibrations, a loud sound and sharp yaw signalled a potentially catastrophic in-flight breakup of the TR assembly. They successfully autorotated to a nearby field (power-on, followed by emergency shutdown of both engines at about 1000 feet). The accident could well be labelled ‘Miracle on the Godavari’ given that the helicopter was at over 90% of MTOW, force landed close to river Godavari, all seven occupants walked away and almost 80% of the helicopter could be recovered back to service.
True to the lyrics of an old Dire Straits song “this is my investigation, not a public inquiry”, the complete story will be known only to HAL. But ‘dire straits’ pretty much summarises the situation should such a failure occur in an unalerted state. Incidentally, 2005 was ‘Year of the Rooster’ as per Chinese calendar. The rooster is a flightless bird.
Mi-17V-5 High Altitude Paradrop. In Oct 2017, an IAF Mi-17 V-5 helicopter was undertaking high altitude ‘air maintenance’. During such sorties, loads secured onto skid boards are paradropped through the helicopter’s rear clamshell doors. One of the parachutes failed to deploy and ‘candled’ into the tail rotor, shearing it away. A deadly combination of high altitude (in excess of 15000 feet), insufficient height above terrain and low speed (airspeed washes off rapidly due rotation) claimed the helicopter and seven lives in less than ten seconds (read it here).
These three accidents lie at the extreme end of the tail rotor troubles spectrum. In between, many combinations of partial control, pedal stuck at high/low pitch, TR actuator runaway, loss of TR effectiveness, etc can catch you unawares.
Tail Rotor Control Failure or Stuck Pedals
Seaking Mk 42B. Year 2003. A Seaking Mk42B helicopter on final approach was entering transition onto the aircraft carrier deck. As the pilot on controls came up on collective on short finals, the helicopter started to yaw right (indicative of TR thrust less than trim thrust, or yaw control stuck on the low pitch side). In no time, the aircraft had lost more than 90º and still rotating. Within three complete 360º turns, the giant helicopter struck water (miraculously clear of the carrier) and disintegrated into pieces. It was all over in seconds. The four crew members escaped by virtue of their underwater egress training. Subsequent investigation indicated possible rupture of the TR control cable or associated mechanism (no details in open media). The survivors – all my good friends, thoroughbred aviators – were ‘four non-blondes‘ and had barely enough time to ask “what’s going on?!”
I reflected on a simulator session where the instructor quietly injected a similar failure on takeoff, just after reaching cruise power where the TR trim thrust is close to ‘pedal neutral’. Though I felt something just that wee bit amiss, all subsequent turns were to the right where left rudder (power rudder on AW139) remained out of play. Reality struck on short finals when increasing power demand overcame the rudder trim thrust.
Darn! I should have gone around instead of trying to make a perfect centreline landing. The resultant vicious spin that follows is most disorientating and best left to simulators. The SFI made his point. When it comes to rudder control, don’t leave any lingering doubts; maybe even make it a practise to check rudders before takeoff and landing. In modern, fully automated helicopters where your feet remain mostly off the rudder pedals to allow ‘George’ (autopilot) to do his job, anomalies can go undetected until it’s too late.
Awareness, Height and Speed is Key
What saved the Dhruv was as much design as it was piloting skill and aeronautical decision making (ADM) – all crucial to handling critical emergencies. The Dhruv is a powerful beast designed to deliver high-performance and handling qualities all the way from sea level to super high altitude. It has large endplates on each horizontal stabilizer that offload the TR in cruise. Cruise flight also presents more reaction time to recognise and take decisions / actions to handle complete loss of TR control. But it takes nothing away from the sheer skill and ADM of the TP from HAL (and his copilot with low on-type experience) who flew the bird to the ground safely minus the engines.
Don’t Ignore Warning Signs
The larger point here is that tail rotor troubles may or may not give you warning signs. Neither may you have the luxury of time and hindsight. Key lies in being sensitive to telltale indications and use the small window available before control is lost. Even in cruise flight, torque reaction can quickly build up large sideslip angles that may lead to undesirable cross-coupling, wash off speed rapidly, dump translational lift and precipitate the situation unless prompt and positive control is exercised.
AS365N Dauphin, 2018. Want an example? Ask Capt Savita Singh, a former IAF pilot, who experienced a TR control failure while evacuating two seriously ill patients from Androth in the Indian atoll of Lakshadweep to the mainland in Mar 2018. Her 160 Nm oversea flight on VT-ELD, an AS365N Dauphin of PHL, was uneventful till she joined right base for landing at Runway 27, Kochi International Airport. Since right rudder is power rudder on the Dauphin, she noticed a marked anomaly during the decelerating turn. In no time, the aircraft had lost 40º-50º direction and couldn’t align with the runway. What followed was a copybook handling of TR stuck at low pitch. Capt Savita went around, bought time, height, speed and airspace, carried out her handling checks, briefed her low-time copilot, went through the checklist “at least 4-5 times” and returned to land “nose straight, no drift” on the centreline at 80-85 kts with almost NIL damage to aircraft or its occupants (DGCA report is awaited).
She makes light of her feat (possibly first on a Dauphin N) with just two words – ‘recognise’ & ‘accept’. Rest, she says modestly, was training, ‘God’s grace’ and then some. Helicopter pilots are no Sullies and a good day is one where you can return home keeping your job. I doff my hat to the feisty young lady and her able first officer Saleh Patel.
Remember, the physics of height & speed and their ‘exchange rate’ is inviolable. Observe advisories like ‘Fly Attentive’ and ‘Fly Manually’ in Rotorcraft Flight Manuals (RFM) diligently. Also remember to lock that harness when and where prescribed in checklists. You don’t want to hit the ground or water at over 360º per second with an unlocked harness.
Loss of Tail Rotor Effectiveness (LTE)
The phenomenon of LTE is described in FAA Advisory Circular 90-95 as “a critical; low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control.” This circular (read it here) is recommended reading for helicopter pilots. Supplementary information is also available in FAA’s Helicopter Flying Handbook, Chapter 11 and many other online sources (some are referenced below).
Noticeably absent is the mention of LTE in any RFM that I have come across. Manufacturers like to maintain that LTE – like other hazardous conditions of helicopter flight (dynamic rollover, vortex ring, etc – is subject matter to be dealt with during training and not a design deficiency in certificated helicopters (read an example here).
Is LTE relevant only to single-engine helicopters?
An NTSB safety alert SA_062 of 2017 dealing with LTE states that “LTE can occur on all conventional single engine, single rotor helicopters…”. The inclusion of ‘single-engine’ raises questions in my mind, as it must in others who have done some reading into LTE. While NTSB data may indicate that most LTE cases have occurred on single engine helicopters, bigger, twin-engine helicopters aren’t immune to it either.
Tail rotor shares its power requirement with the main rotor. Unlike the main rotor which has a speed governing mechanism through the power turbine governor, TR speed is set through MGB-TGB gear ratio by the manufacturer. The pitch change mechanism of TR allows for a range of pitch angles decided by the anti-torque requirements for manoeuvering at certificated max AUW. However, pitch alone doesn’t decide the TR effectiveness since it operates in the wake of main rotor.
TR is required to keep the helicopter fuselage from rotating opposite to main rotor rotation (i.e. left rudder on helicopters with American rotation, anticlockwise when viewed from cockpit). To counter any uncommanded yaw to right, the pilot will use more power rudder (left rudder in this case). This causes greater power offtake from the gearbox which the powerplant must field. On a single engine helicopter, if this exceeds the only engine’s capacity, main rotor RPM will droop. This in turn will droop the tail rotor RPM as well, thereby leading to a deteriorating situation for LTE.
Compare this to the TM333 turboshaft engine fitted on the twin-engine Dhruv that is also configured for yaw pedal anticipator inputs; or the Gnome engine on Seaking where there is an increase in NF (power turbine or N2 RPM) with increasing power, and it is not hard to understand why the NTSB called out single engine helicopters for LTE.
Also contributing to the vulnerability of helicopter pilots to LTE is their almost non-existent training for recognising & counteracting LTE. Simulator models are not robust enough neither have I found it included in recurrent training or check rides. Often, instructors go through spot turns at hover like it is some ritual to be completed rather than point out the nuances of wind angle versus helicopter reaction. That’s a missed opportunity every six month which you could encash in the next check ride!
Which way to turn at hover?
Any turn towards torque reaction (to the advancing side) must require more than usual anti-torque to stop, particularly if the turn rate is allowed to increase unchecked. When winds are quartering, the tail tends to seek the wind. Sudden, opposite pedal inputs can stall the TR (no inflow) or put it into vortex ring conditions. Hover turns are therefore preferable towards the direction of main rotor rotation so that it can be arrested using torque reaction rather than rudder pedals.
Much as we would like to see pedal inputs as left, right or neutral, in actuality, yaw control is calibrated from full low pitch to high pitch with the neutral position roughly corresponding to TR trim thrust at an intermediate condition where the TR is more or less offloaded.
To provide good handling qualities throughout the flight envelope, some amount of control harmony has to be built in to the system. Good ‘control harmony’ means that a similar effort is required to achieve a response in each axis of control (R, P, Y). Thus pedal forces are usually higher than stick forces (maybe we can kick better than we can arm wrestle?). Hence, any reluctance to not use full pedal inputs when required should be guarded against. Onset of LTE should be quickly identified and countered with maximum rudder, as the FAA advisory circular and NTSB safety alert both have correctly noted.
Which Side does the helicopter weathercock?
The FAA’s helicopter handbook describes the aft sector (relative 120º-240º) as the weathervaning sector for conventional helicopters. However, many pilots insist that the helicopter nose weathercocks into wind. This is probably borne out of deteriorating handling qualities and increased pedal activity when winds are on the advancing side or quartering. The helicopter is like a suspended windvane with tail empennage aft of the main rotor shaft. This would cause tail to seek the wind more than the nose, though both would like to present the least resistance to relative winds.
Positive directional stability
ADS 33 requires that for low speed manoeuvres involving banked turns, the aircraft heading remain reasonably aligned with the direction of flight without ‘complex coordination of … controls’. This is achieved by good directional stability – something that deteriorates with quartering winds.
Next time you do a spot turn at hover, notice how the tail accelerates into wind when winds are quartering. Some helicopter RFMs carry specific advisories for tail wind takeoffs and landings. Remember, any behaviour of the aircraft that tends to ‘depart’, thus requiring a counterintuitive input, is destabilising and best avoided.
Hope this write-up encourages you to read further into your aircraft manuals, discuss with your CFIs, utilise your Sim sessions better and, more importantly, respect ‘that thingy at the back’!
Happy landings! Keep your tail clear!
©KP Sanjeev Kumar, 2016-19. All rights reserved.
Disclaimer: If you are a flight crew, please consult national regulations, the Rotorcraft Flight Manual or Aircraft Flight Manual and your company’s Operations Manual as applicable to the type you fly. Views are personal. I can be reached at firstname.lastname@example.org.
For the record, I fly the AW139 quite regularly, as many of you must do too. The cover photo used is for representative purpose only and I don’t mean to undermine this great machine or Leonardo’s family of helicopters. Only the lessons are important, not the type or mark.