To the Letter

Too Heavy: Helicopters Should Not Operate at OGE Hover Limitations

Cartoon of helicopter operating at excessive weight.

Helicopters have been operating too close to the limits for years, and a dissenting point of view has long been wanting. The circumstances under which a helicopter will operate only in ground effect (IGE) are rare. It may happen with scheduled operations when the helicopter is operating from helipad to helipad using paved or concrete surfaces over which ideal IGE hovering conditions are achieved. For those machines operating under just about any other circumstances, out of ground effect (OGE) operations are the rule rather than the exception.

I will use the performance of a common single-engine turbine helicopter as an example. The charts for the OGE hover ceiling for this type, as with any other type of helicopter, are published in its flight manual. These ceilings are predicated on density altitude (DA) in otherwise standard atmospheric conditions in which the humidity is zero percent1. If the pilot is operating at those limits, any attempt to manoeuvre with any application of power, engine and/or transmission limits will likely be exceeded. If operations under these conditions are to be practically and safely carried out, in my opinion gross weight should be less than the maximum specified by the OGE hover ceiling charts to allow the pilot a margin of power for manoeuverability.

To validate my point, I calculated the length of time required to vertically climb 100 ft at specific weights and using the power required to hover OGE. (My calculations were derived by considering the difference between the weight of the helicopter and the thrust required to maintain an OGE hover.) My calculations showed that when operating as close to 50 ft below the OGE limits, it can take up to 20 sec to vertically climb 100 ft. It is reasonable to characterize this sort of performance as sluggish at best, and perhaps dangerous at worst. By reducing the gross weight to 100 lbs less than the OGE hover ceiling weight, I found that the situation improves considerably, and when it is reduced by 200 lbs, the time required to climb 100 ft is half that of a 50-pound reduction. Therefore, the advantages of operating at 200 lbs below the OGE hover ceiling when conducting OGE operations are several:

  • the helicopter is subject to reduced power demands;
  • pilot work load is reduced;
  • the time spent in the shaded area of the height-velocity diagram is reduced;
  • the effects of humidity on density altitude and OGE operations are to some extent mitigated;
  • the critical relative wind azimuth area on the OGE hover ceiling chart will almost certainly be avoided;
  • the pilot’s power margin is greater and manoeuverability is enhanced; and
  • some admittedly small savings in flying time may be achieved in some cases.

Operational helicopter flying involves landings on ridges, pinnacles or helipads constructed on mountain sides or man-made structures, which almost always results in OGE situations. The modern approach to safety is a proactive one and standard practices are designed to reduce the probability of an occurrence. As I attempted to illustrate in my example, gross weights should be reduced by up to 200 lbs below the OGE hover ceiling limits for all but the most mundane of operations.

Fred Lewis
Medicine Hat, Alta. 

Ligne décorative

1 Humidity decreases air density and therefore decreases OGE hover ceilings. At 6 000 ft pressure altitude on a 20° C day with 0% humidity, the density altitude is a bit over 8 000 ft. Under the same conditions and 100% humidity, the DA is almost 8 300 ft, resulting in an OGE hover ceiling weight reduction of about 50 lbs.

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