Max Range Considerations
Running the ECS will burn between 16 and 24 lbs. of fuel per hour. Our flight manual max range charts account for “the worst case ECS condition.” Flying with the ECS off will save approximately 100 lbs. of fuel over the course of a five-hour mission.
Engine anti-ice burns about 84 lbs. of fuel per hour. Use of the engine anti-ice and the resultant increase in fuel burn is not factored into our max range charts. If environmental conditions necessitate the use of engine anti-ice, not planning for its employment can be a critical mistake. The use of engine anti-ice will add 400 lbs. of fuel burn to a five-hour mission. The Bingo Profile Maximum Range Charts quantify this as 8% less range (“multiply tabulated air range by 0.92”).
Open doors and windows create drag, increasing fuel consumption. Like the engine anti-ice situation above, the additional use of fuel is unaccounted for in the performance charts, which utilize a baseline drag configuration with the helicopter’s cabin door and window closed. With the cabin door open for a 500 nm flight, the helicopter will burn about 250 lbs. of additional fuel.
Helicopters’ max range altitude is a compromise between the most efficient altitude for the engines (way higher than a helicopter can fly) and the most efficient altitude for the rotors (sea level). Subsequently, the max range altitude increases as gross weight decreases. A CG MH60T at max gross weight has the best range at approximately 4000’ pressure altitude. But, by 20,000 lbs., max range altitude increases to about 8000’. Changes in altitude adjust the max range indicated airspeed. Changes in weight also change the max range indicated airspeed.
More important than max range altitudes are winds. A 1000’ return to the shore with a 15 kt tailwind may save more fuel than an 8000’ return in calm winds. Meteorologists state that the boundary layer, the layer of air that is subject to surface friction and the Coriolis effect, is generally between 1500’ and 3000’ AWL/AGL. Above 2000 to 3000 AGL, air flows parallel to the isobars. Below these altitudes, friction with the surface forces winds across isobars and then generally towards low-pressure areas. A climb or descent into or out of the boundary layer can shift your winds up to 90 degrees. Consequently, a headwind at 500’ AWL may turn into a crosswind at 4000’, saving a significant amount of fuel over the course of a long-range mission.
Pay attention to the max range airspeed adjusted for winds. Max range airspeed will be faster into a headwind and slower with a tailwind. Airspeed adjustments minimize the time flying into a headwind and maximize the time flying with a tail wind. It is important to note that if a pilot sees the same wind throughout a mission (headwind one way and a tail wind the other way), the helicopter will burn more fuel than the same “zero” wind mission. Because the helicopter is flying an airspeed other than its max range airspeed, there will always be a loss of efficiency. The greater the winds, the more fuel it will take to affect the mission. CAAS illustrates this concept nicely. If a no wind flight 250 nm offshore directly back to the point of origin in CAAS for zero winds is planned, then compared against the same mission with winds (10 kts, 25 kts, 50kts…), the fuel burn increases significantly even if the max range airspeeds for the winds is adjusted.
To achieve the most efficient long-range profile, the general rule is to minimize time in the climb and maximize time in the descent. Pulling 105% torque and climbing at the best rate of climb airspeed (bucket) allows the pilot to minimize time in the climb and maximize time at the max range altitude. On the descent, keep the max range airspeed and plan for approximately a 300 fpm descent. This rule is apparent in commercial flight, as airliners fly at max continuous power in the climb to altitude with a 30 min descent, provided ATC authorization.
CAAS and the performance charts in the flight manual do not include climbs and descents in the fuel planning. Because a climb to 8000’ followed by a descent back down to sea level costs about 20 lbs. of fuel compared to flying the same lateral distance at sea level, if the fuel saved at altitude is more than 20 lbs., pilots benefit from the climb. Reference – there is none. An explanation of these numbers follows the end of the article.
Turbine engines run most efficiently in a steady state. If the altitude hold is continually chasing itself and torque is fluctuating as a result, consider securing altitude hold. Instead, set max range torque and maintain altitude and airspeed with subtle changes in pitch. Although difficult to quantify, a consistent torque setting saves fuel.
Lastly, I want to discuss the widely accepted myth that bringing an engine back to idle saves fuel when in extremis. CAAS cannot help in this situation. Only the performance charts allow pilots to “do the math.” At sea level with a gross weight of 16,500 lbs. on a no wind day flying max range airspeed, the helicopter burns about 910 lbs. of fuel per 100 nms. dual engine. Single engine at sea level flying max range airspeed, the helicopter burns about 690 lbs. of fuel per 100 nms. But, because the other engine is operating at idle, it is consuming about 130 lbs. during 100 nm transit at max range single engine airspeed (100 kts at sea level weighing 16,500 lbs.). Therefore, the combined fuel burn of the two engines is 820 lbs. per nm, which is only 90 lbs. better than if both engines were operating normally (with the engine power control levers in the “fly” position). If truly in extremis, pilots must shut down one of the engines to appreciably save fuel. Completely shutting an engine down saves about 220 lbs. per every 100 nm compared to the same dual engine fuel burn. However, if an engine is secured to increase max range, the pilot must fly the appropriate max range speed (it’s slower single engine) and know that there will be about 200 lbs. of unusable fuel in the main fuel bladder on the side of the engine not being utilized, due to the height of the interconnect between the mains.
Detailed planning, risk assessment and mitigation is essential before departing on a long-range mission. The considerations above have been shared to allow pilots to “make fuel” and land above the planned reserve, not to offer a means of extending the range of a mission. Counting on making a few extra hundred pounds of fuel beyond what was calculated may put the pilots in the untenable situation of having to consider securing an engine to ensure the helicopter makes it back to land.
***Explanation of fuel burn for a climb from sea level to 8000’ followed by a descent to sea level: All I can offer are gouge figures since the charts in chapter 8 do not provide a definitive answer. However, using gouge you can figure to expend about 110 additional lbs. to climb to 8000’ (utilizing the old 60J climb charts) and save about 90 lbs. in the descent from 8000’ (using a 10 percent torque reduction from max range torque on our cruise charts providing an approximate 300 fpm rate of descent).
