We all know it is the pilot’s responsibility to insure the helicopter is flown in accordance to limitations, which in part requires knowing the helicopters takeoff weight. However, due to the versatile nature of helicopters it isn’t always as simple as back in flight school. We may find ourselves picking up a sling load such as a log. How does one weigh a log? A dozen passengers off a ship. Ever try to use a scale on a heaving ship? Or out in the bush of Alaska picking up crew and equipment.,What scale? One aspect that goes along with flying the ultimate off-road vehicle is that we may find ourselves in places without scales.
If conducting an external load operation and the aircraft has a load meter installed, the pilot simply monitors the gauge as the load is lifted A load meter is basically a scale, which measures the weight on the cargo hook. Prior to attempting the lift the pilot should do some quick math to determine the maximum allowable load, which must not be exceeded. This maximum allowable load is the aircraft maximum gross weight subtracted by the aircraft actual takeoff weight without the external load. When hovering over the load, the pilot slowly increases collective, and tension is gradually increased on the sling. The load gauge is monitored to insure it does not exceed the maximum allowable load, and the helicopter will not exceed its maximum gross weight. In this case the center of gravity is not a concern, as cargo hooks are positioned longitudinally to not appreciably affect CG. If the CG was calculated to be good without the load, it should be good with the load.
Most helicopters are not flying sling loads nor have a load cell installed, so we need another method of weight verification. Fortunately some performance charts can be used for this purpose. Performance charts are predictive, enabling a pilot to accurately determine variables prior to takeoff and many can be used in a variety of ways depending on which variables are known. The Sikorsky S-92 flight manual makes this an easy process, with the Indicated Torque Required to Hover in Ground Effect chart. One can predict what the indicated torque per engine will be for a specific weight, density altitude and wind condition. In this example, a negative 3,000-foot density altitude with a 10-knot headwind would equal 66 percent per engine torque for a gross weight of 23,000 pounds. For the same density altitude and wind condition, 83 percent per engine torque would indicate the maximum gross weight of 27,700 pounds is being exceeded. Using the chart it’s easy to see that that 1,000 pounds is equivalent to about 3.5 percent per engine torque, for a given density altitude and wind condition.
If the aircraft lacks this type of chart, a little m,ore work is necessary. The takeoff and maximum continuous power Hover in Ground Effect charts also provide maximum weights for a range of density altitudes. This gives a start for making your own quick reference chart, and after a couple dozen flights you can add more data points with other power settings. Say you flew 500 lbs under gross weight with a 1000-foot density altitude; simply note the torque in a stable in ground effect hover and enter the torque, density altitude, and weight on your quick reference chart. Over time, you will have created a chart to use as an aid when you are unsure of the aircraft takeoff weight. An external load pilot, without a load cell may opt to use a HOGE (hover out of ground effect) chart instead of a HIGE chart. Experienced pilots with a lot of time-in-type already have a pretty good idea of the power required for specific weights and density altitudes, which is essentially what this quick reference chart provides.
The pilot should also note the cyclic position necessary to maintain a stable position over the ground, providing an indication of the aircraft’s center of gravity. An excessive lateral or longitudinal deviation from a normal position can indicate a CG out of normal range. Wind can also effect the cyclic position, but experience in type will help you learn what a normal cyclic flight control position should be in a variety of conditions. For example, a farther forward and left cyclic position than normal would indicate an aft and right CG, which a left quartering headwind could also cause.
These methods are certainly not a substitute for a proper weight and balance calculation using accurate weights. They are a means of verifying your calculations, particularly when in situations where the weights provided may be in question. It is also a means of understanding the performance of your helicopter better.