Given: Economy @ FL100 ISA — Page 68, Lesson 71

I want to walk you through the cruise power setting and fuel flow data for a multi-engine piston aeroplane, or MEP, using CAP 697 Figure 3.3. This is the table you'll use to set up your cruise power and work out your fuel consumption. First, the power settings are expressed as percentages of maximum continuous power. There are four standard settings: 75% which is called HIGH SPEED, 65% which is called ECONOMY, 55%, and 45% which is called LONG RANGE. So if you see "Economy" in a flight planning question, that means 65% power. To get your true airspeed, or TAS, you do not read it from this power setting table. Instead, you use a separate graph called the SPEED POWER table, which is Figure 3.4. I'll come back to that in a moment. For fuel flow, you enter the table with the required percentage power. The fuel flow is given in US gallons per hour, abbreviated as US.gal/h. For example, Economy Power at 65% has a fuel flow of 23.3 US.gal/h. If you have a time, say 2 hours 30 minutes, you can work out the trip fuel. 2 hours 30 minutes is 2.5 hours, multiplied by 23.3 US.gal/h gives you 58.25 US gallons of fuel for that leg. The manifold pressure is read off against pressure altitude and rpm in the correct percentage power column. So for a given power setting, you look up the pressure altitude and the rpm you plan to use, and the table gives you the manifold pressure in inches of mercury, inHg. Here's an example from the note. At 75% power with a fuel flow of 29.0 GPH — that's gallons per hour — at FL60, you can achieve that with either 33.4 inHg at 2500 rpm, or 32.2 inHg at 2600 rpm. So you have a choice of rpm, and the manifold pressure adjusts accordingly to give the same power. Also note that as power decreases, fuel flow decreases. That gives you greater range and endurance at lower power settings, which is why 45% is called LONG RANGE. Now, there is a temperature correction for manifold pressure when the outside air temperature differs from ISA. For each 6°C above ISA, you add 1% to the tabulated manifold pressure. For each 6°C below ISA, you subtract 1% from the tabulated manifold pressure. So if you're flying in hotter than standard conditions, you need a slightly higher manifold pressure to get the same power, and in colder conditions, you need slightly less. Unlike the single-engine piston, or SEP, power graphs, the TAS is not extracted from this table. So a separate graph is provided, which is Figure 3.4, the True Airspeed graph for the MEP. You use this graph to obtain the TAS for various combinations of temperature, altitude, and percentage power setting in the cruise configuration. Let me give you some examples of TAS extraction from Figure 3.4. These are the answers you would get from the graph. For High Speed at FL120 ISA, the TAS is 183 knots. For Economy at FL120 ISA, the TAS is 178 knots. For LRC — that's Long Range Cruise — at FL120 ISA, the TAS is 146 knots. So you can see the trade-off: higher power gives higher speed but higher fuel flow, and lower power gives lower speed but better fuel economy. You also have examples with temperature variations. For Economy at FL80 with an OAT of plus 20°C, and for Economy at FL80 with an OAT of minus 20°C. The graph allows you to read the TAS for those non-ISA conditions as well. So in summary, for MEP cruise planning: you use Figure 3.3 to get your manifold pressure and fuel flow for a given power setting and altitude, applying the ISA temperature correction to manifold pressure. Then you use Figure 3.4 to get your TAS for that same power setting, altitude, and temperature. With TAS and fuel flow, you can then work out your time, distance, and fuel required for each leg of the flight.