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8.7 Helicopter Limitations

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Hover Performance

A helicopter is considered to be hovering under the following conditions; position and hight over a surface remains constant with rotor rpm and heading. The factors effecting hover performance are aircraft weight and air density. The hove ceiling is reduced when air temperature and/or weight is increased.

A helicopter can have a hover ceiling higher in ground effect than out of ground effect due to the fact a mountain can provide a ground cushion. Ceiling is defined as a hight in the international standard atmosphere (ISA). A ground cushion IGE requires less power to hover than no ground cushion hovering OGE under fixed atmospheric conditions.

There are two hover performance charts in the Bell 206L-1 Long Ranger II Handbook, one for hovering in ground effect (IGE) and the other for hovering out of ground effect (OGE) on pages 16 and 17 using Take-off Power. Both have the particle separator purge switched off and anti-ice off and both graphs use the same method to determine hover performance.

Relative Wind

Area “A” (the white squares) as shown on the hover ceiling charts presents hover performance for the BELL 206L-1 that demonstrates stability in relative winds of 17 knots sideward and rearward at all loading conditions.

Area “B” (shaded squares) as shown on the hover ceiling charts presents additional hover performance which can be realized in clam winds outside the critical relative wind azimuth area.

In other words, if the wind is coming from the azimuth area relative to the helicopter, do not use the additional hover performance Area “B” (shaded squares) on the hove charts.

Refer to the BELL 206L-1 LONG RANGER II CASA HANDBOOK for an example with instructions on pages 13, 14 and 15 about using the Hover IGE and OGE graphs on pages 16 and 17.

Calculating Hover Weight
  • Calculate the pressure altitude if the QNH pressure is different from the ISA pressure of 1013 hPa.
  • Start by entering the graph from the temperature scale, located at the bottom left hand corner. Then move vertically up to intersect the pressure altitude.
  • Where the temperature intersects the pressure altitude, move horizontally right to intersect the other temperature line (use the same temperature).
  • If the wind is from the critical azimuth (Refer to handbook, page 14) remember to stay out of Area B stoping you from reaching the other temperature line in Area B.
  • Move vertically down to read the maximum weight for hovering OGE.
Hover IGE Weight

To find the maximum gross weight for hovering in ground effect, use the hover IGE chart (page 16) with the following data. The OAT is 20° while operating at a pressure altitude of 10,000 ft in calm wind conditions.

Answer 3900 lb

If wind was blowing into the azimuth area, stop at the area “B” boundary line.

Answer 3450 lb

Hover OGE Weight

To find the maximum gross weight for hovering out ground effect, use the hover OGE chart (page 17) with the following data. The OAT is 20° while operating at a pressure altitude of 10,000 ft in calm wind conditions.

Answer 3300 lb

If wind was blowing into the azimuth area, stop at area “B” boundary. (Coincidently the area “B” boundary line and the 20° line overlap providing the same answer 3300 lb).

Calculating Hover Ceiling
  • Decide which graph to use HIGE or HOGE based on skid height. (A sling load requires the out of ground effect graph.)
  • Start with gross weight and move vertically up to intersect the OAT line. Remember to stay clear of Area B if wind is blowing from the critical azimuth (page 14).
  • Move horizontally left to intersect the vertical temperature line coming up from the bottom. (The vertical temperature line is the same OAT used previously.)
  • Read the maximum Pressure Altitude (Pressure Height) at the intersection.
  • Now consider the QNH (pressure at sea level). If the pressure difference is greater than 1013 add the difference to your Pressure Altitude intersection for the correct elevation. More pressure than 1013 hPa means better performance, higher on the graph. Conversely should the pressure be less than 1013 poorer performance will be expected, move lower on the graph.
Adding to Pressure Altitude

If the pressure is greater than 1013, add the difference to the P.A. intersection point to find the elevation.

  • QNH 1033 hPa
  • Skid Height 40 ft
  • Gross Wt 4050 lb
  • Wind Calm
  • OAT 10°

(1033 - 1013) 30 = 600ft difference.

Now add the difference (600 ft) to the intersections of the P.A. (6000 ft) to find an elevation of 6600 ft.

Subtract from Pressure Altitude

If the pressure is less than 1013, subtract the difference from the P.A. intersection point to find the elevation.

  • QNH 993 hPa
  • Skid Height 40 ft
  • Gross Wt 4050 lb
  • Wind Calm
  • OAT 10°

(993 - 1013) 30 = -600 ft difference.

Now subtract the difference (600 ft) to the intersections of the P.A. (6000 ft) to find an elevation of 5400 ft.