Fuel, Distance and Time to Climb
A final consideration related to fuel consumption relates to the fuel required to climb to altitude. While climb fuel consumption rates are insignificant in the case of short or brief climb segments, climb fuel becomes very significant in the case of extended or prolonged climb segments. It is considered generally accurate to plan fuel requirements for a trip with the assumption that the fuel burn rate during the climb segment is double that of fuel burn rate during cruise.
In the case of the Cherokee, the Pilot Operating Handbook does not provide any definitive data on the rates of fuel consumption during a climb. We could extrapolate from the rate of consumption for 75% power (8.4 GPH) and say that the 100% power would be 10.3 GPH (25% of 8.4 is 2.1, and this could be added to the 8.4 figure), but there is no direction from the manufacturer to do this, and it leaves us uncomfortably below the climb rates of fuel consumption one might come to expect based on experience with piston aircraft. While the 75% power rate of 8.4 GPH is based on best economy mixture settings, aggressive leaning to this extent during a climb could cause engine damage.
To resolve the climb fuel consumption issue in the case of the Cherokee, we assume double the cruise consumption rate of 8.4 GPH—so the climb rate is 16.8 GPH—and we determine the time required to climb using the climb performance graph similar to that appears below.
In using this graph, two rates of climb are determined, one being the rate of climb expected at the bottom of the climb—in the example show, from an airport located at an elevation of 2500’—and the other being the rate of climb expected at the top of the climb—in the example, a cruising altitude of 9500’ is planned. From these two rates, the average rate of climb is calculated—e.g., 340’ per minute. At this rate, it will require approximately 20 minutes to complete the climb. Once this time-to-climb is known, the pilot can determine the fuel required (based on, in the case of Cherokee, 16.8 GPH), and the distance flown (based on 20 minutes at a speed of 74 NM).
It is only after we make such climb calculations that we can then make determinations for the fuel and time required for the cruise portion of the flight, since the distance to be covered in cruise flight is the total distance from the departure airport to the destination airport, less the distance required to climb. As a rule, allowance for the effects of wind is not factored into climb speed.
The Cherokee method for calculating fuel, distance, and time to climb is relatively simplified in that it is based on one figure—the aircraft’s rate of climb. Many aircraft Pilot Operating Handbooks, however, present a more detailed and accurate graph which the pilot uses to do the math. The graph which appears below in based on the climb performance chart found in the Piper Lance, a big sister for the Cherokee 140 that seats seven people, has retractable gear, and is pulled along by a big 300 horsepower engine (the Piper Lance achieves speeds during flight that are similar to the twin engine Piper Seneca).
As can be seen from this graph, the pilot has to calculate the fuel, distance and time to climb from sea level to the proposed cruising altitude; from these values, the pilot must then subtract the fuel, distance and time required to climb from sea level to the height of the departure airport.
The calculations which appear on this graph show the results for a planned climb to 10500’ from a airport of departure with an elevation of 10500.’