Saturday, August 1, 2009
Military Spin: Old School Fuel Planning
Computing available fuel for contingency mission execution is an art lost to the technology of mission computers. Sometimes having a simple tool to pull from your mental toolkit or from a page in your kneeboard can be a godsend during time-critical mission tasking.
When tasked for a mission that includes a search, it’s paramount to properly calculate all the segments of the search and rescue which consume fuel. Clearly, you must calculate the time and fuel consumption for the enroute portion of your mission. Based upon mission criticality and/or available onboard fuel, this may be flown at maximum continuous speed V H or maximum range speed V M/R. A commonly omitted allowance is for sighting checks. Deviations from planned search patterns to confirm or ignore a possible sighting often consume 5 percent of total fuel onboard during a search. Remember to include fuel for the survivor landing (or hoist) and recovery. Some turbine aircraft burn substantial amounts of fuel even at flat pitch while waiting for the medics to package your patient. Some aircraft can’t be throttled back to idle without bringing an APU online which then consumes a large portion of the potential idle RPM fuel savings. Knowing and using your most fuel efficient idle mode is essential to saving fuel and maximizing support time. Knowing your bingo fuel helps make the idle versus shutdown decision. The next enroute leg is to the hospital or recovery area. Again speed is dependent upon patient criticality, available fuel and distance. Remember to calculate the distance from the farthest point in the search area as Murphy always seems to put you there when you need gas or find your survivor. Sometimes you’ll simply have to fly at V M/R to have sufficient fuel reserves to make it to the delivery site. Delays at the hospital helipad are inevitable. From the delivery point you’ll have to either get to a fueling site or home. Finally, civilian and military regulatory guidance provides the requirements for your VFR or IFR fuel reserve. What results is a simple kneeboard spreadsheet format on which you have all the consumables in the top box in both pounds and hours and the available fuel on the bottom from which you subtract the sum of the consumable segments. The fuel remaining after these segments is that available for the search. The planner then has to determine at what speed he’s going to fly his search pattern to maximize coverage. A good rule of thumb is to use recommended cruise speed for preliminary route, parallel or creeping line searches, and max endurance V M/E speed for the concentrated search portions. Establish a Joker fuel level that triggers a crew decision point for search continuation or a refueling stop.
One obvious need with this manual method is a quick means to measure time, distance and heading for the initial divert and for the various segments without using the navigation computer. Even the most advanced flight management systems don’t have the flexibility to quickly calculate segment fuel without manually entering several waypoints and running the fuel calculator. Keying in several lat/long takes a long time and using a plotter and a whiz wheel is a bit of a pain. I carry in my flight bag, what’s called a Declitractor. I’ve made a local modification creating a flexible laminated plastic plotter scale that’s pinned in the center of it. This scale has distance and time (at nominal V M/R speed) measurements with 1:500 sectional scale on one side and 1:250 JOG on the other. You drop the device centered on your present position, align the declination grid (adjustable for agonic line changes), pull the plotter scale to your destination to obtain time and distance, and read the plotter line against the compass for proper magnetic heading. Quickly put these times into your fuel chart and compute the appropriate fuel consumption for each segment based on required speed. The chart provides a quickly calculated loiter/search time for your mission, taking the guesswork out of search and divert fuel planning.
Search & Divert Fuel Calculation Sheet