Anthropometric Parameters for IAF Helicopter Pilots

Methods

The anthropometric measurements of 64 cadets in the age group of 20-23 years, who had been allotted the helicopter steream after completion of basic flying training, were analysed using Microsoft Excel® software. Of these, three (03) cadets who corresponded to the lowest, middle and highest percentiles in terms of stature height, leg length and thigh length were seated in the Chetak cockpit to determine their ability to reach and operate all controls and panel switches. The various reaches of these cadets were analysed in conjunction with the ergonomic layout of the aircraft.

Results

The mean height of the cadets was found to be 174.7 ± 24 cm. The average leg length was 108.3 ± 9.8 cm. The mean thigh length was found to be 60 ± 4.6 cm. The percentile values of the three subjects in terms of all the three parameters are given in Table: 2.

All cadets had adequate clearances from all aircraft structures. None of the cadets had any problem in reaching the main instrument panel of the aircraft with the seat harness fully tightened; functional arm reach of all cadets was found to be adequate. The tallest pilot and average statured pilot were able to operate the rudder pedals without any difficulty indicating an adequate leg length and thigh length. However, the short statured pilot was found to De experiencing difficulties in operation of the rudder pedals even after using a 5 cm thick lumbar support pad. In addition, when this pilot operated the cyclic control and the brake on the ground simultaneously, he was unable to operate the rudder pedals properly.

Discussion

The IAF has laid down anthropometric parameters for fighter aircrew flying different aircraft. However, there are no such special laid down requirements for helicopter aircrew. As of now, the anthropometric requirements for helicopter aircrew of the IAF are the same as for flight test engineers and WSOs of Su-30s and are as given in Table 1 [2].

The Chetak helicopter has certain peculiarities due to which the problem of aircrew – aircraft compatibility is compounded. The seat pan of the aircraft is inclined and makes an angle of 20° between the thigh tangent line and the bottom tangent line (thigh tangent angle) (Fig. 2).

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Fig. 2:

Thigh tangent angle in the aircraft

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Even though this angle is within the limits specified by the relevant Mil Std [3], this inclination prevents the aircrew with a shorter leg length from extending his/her knee completely for operation of the rudder pedals. The shorter pilot’s knee and ankle joints are relatively extended during rudder pedal operation, even with the rudders in the fully aft position and the seat in the fully forward position, thus leaving little scope for further extension (Figs. 3, 4, 5, 6). Thus, an aircrew with a shorter thigh length and leg length finds it difficult to operate the rudders as compared to one who has longer leg and thigh lengths. The problem is compounded by the fact that the aircraft in question has only fore and aft adjustment of the aircrew seat and no vertical adjustment.

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Fig. 3:

Knee flexion in a tall pilot

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Fig. 4:

Knee flexion in a short pilot

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Fig. 5:

Ankle position in tall pilot while operating rudder pedals

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Fig. 6:

Ankle position in short pilot while operating rudder pedals

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Another problem faced by shorter aircrew is an inability to rest their right arm on their right thigh while flying without lumbar cushions. This position is known as a “spot weld,” (Fig. 3) and is usually considered necessary to achieve stability to fly [4]. Rotary-wing pilots are subject to continuous vibrational stress during flying and tend to fly in a crouched position [5, 6], which is almost universal due to their desire to place the right forearm on the right thigh. This “spot weld” is necessary to hold the cyclic control firmly while undergoing vibration. Inability to keep the right forearm on the right thigh during flying results in jerky motion of the cyclic control (Fig. 7).

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Fig. 7:

Separation between right forearm and thigh in a short plot

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Moreover, the distance between the control grip reference point and the xiphisternum is also less in the shorter aircrew (Figs. 3, 4). The control grip reference point is the point at which the crew member’s second finger (middle digit) is in contact with the forward or downward face of any grip type control such as the control stick [7]. To compensate for the extended knee, the short aircrew use a lumbar support cushion which shifts the seat reference point [8] forward by 5 cm. This forward movement allows the aircrew to flex the knee by an additional 5 degrees, but at the same time, reduces the distance between the control grip reference point and the xiphisternum by around 5 cm, there by changing the aircrew’s grip on the cyclic control and in turn, making stick motion jerky (Figs. 8,9,10).

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Fig. 8:

Position of cyclic control in aircrew using lumbar cushion

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Fig. 9:

Normal grip on cyclic control

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Fig. 10:

Changed grip on the cyclic in a short pilot

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The brake in the aircraft, which is used during ground taxiing, is located in between the two seats. The same is operated by a pumping action by the aircrew. The brake lever is at a low level, close to the aircraft floor and for operating it, the aircrew has to bend sideways (Fig. 11). Simultaneous operation of the cyclic control and the brake lever does not pose any problems for the taller aircrew. However, in shorter aircrew, it has been reported that this operation of multiple controls, at times results in elevation of the foot off the rudder pedals. It is well documented that the seat and control configuration in most helicopters force the pilot to bend forward in his/her seat and lean slightly to the left [9, 10]. Furthermore, the pilot must maintain this position during most of the time he/she is at the controls, since full control of the aircraft requires simultaneous input from all four extremities [11].

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Fig. 11:

Side tilt during brake operation

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Keeping in view these problems associated with helicopter flying, numerous air forces around the world have defined leg length limits for their helicopter aircrew [4. 12]. However, such parameters have not been defined in the Indian Air Force.

Conclusion

This study shows that the problem of control accessibility in the Chetak helicopter is sufficiently significant to warrant a re-definition of anthropometric requirements for the IA F helicopter aircrew. Institution of a minimum limit far leg length in this regard will go a long way in avoiding unnecessary attrition of trainees during their helicopter flying training and will also substantially help in promoting flight safety and mission effectiveness.

Recommendations

It is recommended that a minimum leg length of 105 cm should be instituted for helicopter aircrew of the IAF. This leg length corresponds to approximately the 30^th percentile of our sample population. Since tall pilots with longer legs have no problems in operating all controls of the aircraft, an upper limit for this parameter need not be defined.