Tab Data

After learning about my cruise chart technique for determining hover performance, many pilots inquire as to why I prefer this method over using tab data for power considerations. Although tab data provides some useful information, it has two considerable shortcomings:

1. Tab data provides pilots with the hover weight they can achieve at the lesser of values – dual engine continuous torque (106% torque) or intermediate torque available (ITA). ITA is 2-3% below maximum torque available (MTA). Therefore, if ITA is the lower of the two values (i.e., the limiting factor), when a crew hovers at the hover weight provided by tab data, the crew has a 2-3% power margin before Nr droops (e.g., TGT limiting prevents the motors from producing any more power). Because a 2-3% margin is well below what is recommended by the flight manual, which warns, “when maximum power available is not at least 10% greater than that required to HOGE, the maneuver becomes very demanding and should not be attempted unless mission urgency dictates,” in most cases, appropriate risk management requires a wider margin. As such, if pilots harness tab data, they must rely on rules of thumb to estimate the hover weight to provide a more suitable power margin (e.g., the 10% minimum margin suggested by the warning in the flight manual). Although subtracting 1000 lbs from the tab data hover weight will add roughly 6% to a power margin of 2-3% and give you something near an 8% margin, determining the appropriate hover weight at the intended power margin using the cruise charts is efficient and more accurate than using tab data and a rule of thumb (e.g., 200 lbs = 1% torque or 1000 lbs = 6%). 

2. When the flight manual’s dual engine torque is the limiting factor (resulting in a hover weight that coincides with 106%), tab data will not help pilots determine ITA. In this case, pilots only know that they can HOGE outside of the10 second torque timer and that the power margin is at least 2-3%; they do not know the specific power margin. For example, at 4000’ and 20°C, tab data informs pilots they can hover at 106% with a helicopter gross weight of 20,600 lbs. To determine that ITA is 112% and MTA is 115% giving them a 9% margin at the weight depicted in tab data (20,600 lbs.), pilots need to refer to the cruise charts. 

Whereas tab data serves as an excellent way to validate/double check TOLD information and inflight performance data (HOV OVERRIDE information), I find cruise charts superior for more efficient, complete and accurate pre-mission planning. 

*** The MH60T community’s 6% per 1000 lbs. is accurate (within a percentage point) at sea level when helicopter gross weights are between 21,500 lbs and 16,500 (our community’s typical gross weight range). However, at higher altitude, the power required to HOGE difference per 1000 lbs is greater than 6%. For example, at a pressure altitude of 6000′ and a temperature of 10 degrees, the difference in power required to HOGE between 21,000 and 20,000 lbs is about 10%. The difference in power required to HOGE between 17,500 and 16,5000 lbs is about 8%. Therefore, at altitude, the 6%/1000 lbs rule of thumb is conservative when determining power required after decreasing aircraft gross weight (e.g., jettisoning fuel at altitude will provide a crew with slightly greater than 6%/1000 lbs decrease in power required to HOGE). However, using the rule of thumb to predict power when taking on weight will result in narrower margins than expected (e.g., hoisting or embarking survivors will increase power requirements greater than 6%/1000 lbs).