TSB .. not the bank .. but the bite!
The Gazelle suffers from a phenomenon known on Aviafora as the 'Tail Swing Bite'!
This condition is also known as ..
- Loss of fenestron authority/effectiveness
- New Gazelle pilot's syndrome
- Not knowing one’s left from one’s right
- Fenestron stall
What is certain is that for those transitioning onto the Gazelle, there needs to be an awareness of this condition, and above all, how to prevent it.
A good start in educating oneself about the Tail Swing Bite, is to review some of the accidents related to this condition:
1) SA341G YU-HEW JAN 2008
Pilot Experience
Total Experience (Helicopter): 56
Total Hours (On Type): 46
Comments from Accident Report
In the absence of any significant technical defect, it is considered that the pilot lost control of the helicopter in yaw due to the strength, direction and gusty nature of the wind acting on the aircraft whilst flying at low forward airspeed.
However, it is considered that the main contributing factors to this accident were the pilot’s lack of experience and probable inadequacies in his training.
However, it is considered that the main contributing factors to this accident were the pilot’s lack of experience and probable inadequacies in his training.
Two fatalities.
AAIB Accident Report
2) SA342J F-GJSL MAY 2005
Pilot Experience
Total Experience (Helicopter): 600
Total Hours on Type: 12
Comments from Accident Report
Pilots who are inexperienced on the Gazelle need to be particularly aware of this apparent loss of tail rotor control. Unlike several helicopter types routinely used for training, the man rotor rotates in a clockwise direction (when viewed from above) and right pedal rather than left pedal is needed to oppose main rotor torque.
Also, the fenestron-equipped Gazelle requires greater pedal deflection than that required for manoeuvring other training helicopters. Additionally, the tail fin is considerably larger than non-fenestron equipped helicopters, leading to more challenging spot turns in windy conditions. In view of these characteristics, the statement in the MoD FM of: "Whenever possible, the first turn should be made to the right to check the maximum rotor torque required.”
Also, the fenestron-equipped Gazelle requires greater pedal deflection than that required for manoeuvring other training helicopters. Additionally, the tail fin is considerably larger than non-fenestron equipped helicopters, leading to more challenging spot turns in windy conditions. In view of these characteristics, the statement in the MoD FM of: "Whenever possible, the first turn should be made to the right to check the maximum rotor torque required.”
Two seriously injured.
AAIB Accident Report
3) Westland Gazelle HT2 G-BZOS JUL 2002
Pilot Experience
Total Experience (Helicopter): 167
Total Hours on Type: 21
Comments from Accident Report
The pilot anticipated the need for right yaw pedal during the takeoff, but just after becoming airborne the helicopter yawed to the left and despite the use of more right yaw pedal, the rate of yaw increased. As the aircraft passed through 180° the rate of yaw was too high to land and the pilot became confused. He applied right cyclic to try to counter the yaw but the aircraft rolled to the right and the main rotor blades struck the ground.
Eurocopter Service Letters as well as the Ministry of Defence Gazelle Flight Manual, emphasise that very high yaw rates can rapidly develop in the hover in light wind conditions with only very small applications of left yaw pedal.
Eurocopter Service Letters as well as the Ministry of Defence Gazelle Flight Manual, emphasise that very high yaw rates can rapidly develop in the hover in light wind conditions with only very small applications of left yaw pedal.
Two persons with minor injuries.
AAIB Accident Report
4) Westland Gazelle HT3 G-BXZE
Pilot Experience
Total Experience (Helicopter): 180
Total Hours on Type: 2
Comments from Accident Report
The pilot reported that the wind was calm and he lifted to a hover about 6 to 8 feet above the ground. He began a left pedal turn to position the helicopter for transition over the open arena area to the east. As the turn approached 180° from the initial heading, the pilot felt that he was unable to stop the helicopter turning left. The rate of turn began to increase
rapidly.
After some 10 to 12 rotations, the pilot elected to land the helicopter immediately while still turning rapidly to the left. On touchdown, the right skid broke and damage was sustained during the rotational deceleration.
EUROCOPTER reminds you that in some configurations (hover flight, flight at low speed in light wind etc), starting a turn to the left can induce a high-rate turn if the pilot does not counter this high-rate turn by applying the correct amount of right yaw pedal.
rapidly.
After some 10 to 12 rotations, the pilot elected to land the helicopter immediately while still turning rapidly to the left. On touchdown, the right skid broke and damage was sustained during the rotational deceleration.
EUROCOPTER reminds you that in some configurations (hover flight, flight at low speed in light wind etc), starting a turn to the left can induce a high-rate turn if the pilot does not counter this high-rate turn by applying the correct amount of right yaw pedal.
One person with minor injuries.
AAIB Accident Report
5) SA341G G-BCHM JUL 1997
Pilot Experience
Total Experience (Helicopter): 68
Total Hours on Type: 29
Comments from Accident Report
The initial lift-off was normal but at a height of about 3 feet the helicopter yawed to the left. The pilot believes that he took the necessary corrective actions with the yaw pedal but the left yaw continued through approximately 2 revolutions with the helicopter climbing to about 9 feet and rolling to the right. The pilot elected to carry out an immediate landing by lowering the collective lever. Ground contact was firm but the helicopter remained upright and came to rest on the original take-off heading.
It is therefore most likely that the helicopter suffered the sudden loss of yaw control which can occur in the Gazelle in light wind conditions.
It is therefore most likely that the helicopter suffered the sudden loss of yaw control which can occur in the Gazelle in light wind conditions.
No serious injuries.
AAIB Accident Report
6) SA341G G-HAVA JUL 1997
Pilot Experience
Total Experience (Helicopter): 170
Total Hours on Type: 25
Comments from Accident Report
The wind was from the left and the intended departure direction was to the west. The pilot was therefore conscious of the need to turn to the left after take off and may inadvertently have applied left pedal during the take off process. The helicopter's behaviour would then have been consistent with the skid marks indicative of it yawing whilst still in ground contact and with witness descriptions of it turning immediately on leaving the ground. Conditions would then have been conducive to the rapid increase in yaw rate experienced in other Gazelle accidents and incidents, particularly if the wind had been modified by adjacent hangars to provide a less favourable local wind component from the right. It is therefore likely that the pilot experienced a sudden loss of yaw control induced by her early left pedal input.
It is therefore recommended that the Civil Aviation Authority (CAA) reconsider the type-rating training requirements for the Gazelle to determine whether additional emphasis needs to be placed on yaw control during take off, landing and low speed manoeuvres.
It is therefore recommended that the Civil Aviation Authority (CAA) reconsider the type-rating training requirements for the Gazelle to determine whether additional emphasis needs to be placed on yaw control during take off, landing and low speed manoeuvres.
Two persons seriously injured.
AAIB Accident Report
7) SA341G G-TURP SEP 1991
Pilot Experience
Total Experience (Helicopter): 108
Total Hours on Type: 9
Comments from Accident Report
The Gazelle has a history of suffering unexplained loss of fenestron effectiveness, commonly known as ‘fenestron stall’. Known previous cases include .. 11 accidents plus 11 incidents in the UK military fleet. The majority of documented ‘fenestron stalls’ have occurred in the hover during an attempted left turn in conditions of wind speeds of 10 kts+ from astern or from the right but there have also been cases in a variety of other wind and flight conditions.
No serious injuries.
AAIB Accident Report
8) SA341G G-SFTA MAR 1984
Pilot Experience
Total Hours (Helicopter): 6,000
Total Hours on Type: 120
Comments from Accident Report
The helicopter took off from Carlisle Airport at 0800 hrs on a navigational training sortie which was to simulate the pick-up of troops at various points on the route. About 18 mins after take-off the student had identified a pick-up point near a barn and was told by his instructor to look for a suitable landing area. They flew past the barn at 40-50 kts with a slight left bank at about 100-150 ft agl. The helicopter started to yaw to the left at an increasing rate with the airspeed reducing. The instructor reached for the controls and asked the student what he was doing. The student replied that he had lost control. By now despite the full right pedal and forward cyclic applied by the instructor the helicopter was rotating to the left and descending. The instructor raised the collective just before the helicopter hit the ground nose down and right skid low. The helicopter, after contacting the ground., rolled over onto its right side.
No serious injuries.
AAIB Accident Report
From these reports it seems to me that the UK’s AAIB and CAA both believe in the existence of ‘Fenestron Stall’ (even if that isn’t the best phrase to describe it).
I think the moral of the story may be to ensure that new Gazelle pilots are thoroughly acquainted with the potential for the Gaz to deliver an unwanted ‘tail swing bite’ .. and for the need to be aware of those conditions which can induce this.
~ ~ ~
TAIL SWING BITE SAFETY TIPS
REMEMBER WHEN IN THE HOVER AND AT LOW SPEEDS
NOT TO PERMIT YAWING TO THE LEFT TO GET OUT OF CONTROL
PERFORM ALL LEFT PEDAL TURNS IN A SLOW AND CONTROLLED MANNER
IN MODERATE TO HIGH WIND CONDITIONS, REMEMBER THAT THE FENESTRON'S RELATIVELY
LARGE SURFACE AREA WILL CAUSE THE AIRCRAFT TO 'WEATHERVANE' AND BE MINDFUL AS TO
HOW THIS MAY AFFECT OR EVEN INDUCE LEFT HAND YAW
WHEN IN DOUBT, PERFORM PEDAL TURNS AND LOW SPEED TURNS TO THE RIGHT