• Kevin Shields President of Trans Aero, Ltd.

Do You Really Know Your Aircraft’s Lifting Capability and Its Limitations?


Know Your Capabilities and your Limitations

Scenario:

You are doing a lift job with little margin. The Load calculation indicates the load is within 200 lbs of the aircraft’s maximum lifting capability for the altitude and temperature. As the lift is attempted, the pilot increases power to 50 psi on the torque meter, the load does not move, and the lift is aborted.

The flight manual supplement indicates that the maximum torque is 50 psi, which translates to 1100 horse power, and we should have been able to lift the load according to the performance charts. So why won’t the aircraft lift what the charts indicate it should be capable of lifting?

The FMS states in paragraph 2-16 and 5-7 that the helicopter is torque limited by the transmission to 1100 HP, which corresponds to 50 psi on the torque gage at 6600 engine/324 rotor RPM. However in paragraph 7-13 there is a notation that states “the power output capability of the T53-L-703 engine can exceed the transmission structural limit (50 psi calibrated) under certain conditions.” The key word here is “calibrated.”

In the TCDS for our aircraft, R00005SE, under note 12, it states “torque pressure output by the torque sensing system varies with the individual engines. The calibration of this value is required on each engine and the value corresponding to take-off power, is stamped on the engine data plate.” This statement applies to both the L-13 and the L-703 engines.

In the installation documents of the 703 engine STC, there is reference to TM55-1520-210-23-1/2/3, which is the UH-1 maintenance manual. In book 2 Section I, paragraph 8-6 (d), there is a procedure for marking the maximum torque limit by using the information on the engine data plate and a conversion chart in that paragraph.

So 50 psi on the torque gauge does not necessarily equate to 1100 SHP. As stated above, the torque sensing system varies with every engine, and therefore every engine will produce more or less power at a specific torque setting, and thus the need for calibration of the torque gage

How do we know how much power our engine is making?

During the engine test cell runs, one of the test points is Torquemeter PSI at 1125 ft/lbs. That is the torque pressure number which is stamped on the engine data plate. Since the power rating is taken at 1125 ft/lbs, the conversion chart is used to mark the torque gage to indicate 1100 SHP, which will usually be slightly above or below 50 psi.

What do I do with this number?

As stated above, TM 55-1520-210-23 requires me to convert this number to an instrument marking on the torque pressure indicator which then becomes the maximum allowable torque, equal to 1100 SHP.

1 psi on the gauge is about 200 lbs of external load lifting capability.

If the gauge is not marked properly the aircraft may not be producing the power necessary to meet the performance charts and lift as much as it should, or conversely it may be producing over the maximum rated power.

If your engine produces more than 1100 shp, there is a possibility of causing structural damage to the transmission as well as the airframe.