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Performance: Optimum Altitude
What is Optimum Altitude, Anyway?
One of the less-understood settings that frequently gets ignored is the Optimum setting for the initial cruise altitude. Most platforms including NZ, EPIC, and NG FMS have a default selection of Optimum as the cruise altitude entry. After observing most flight crews override optimum with their filed flight plan altitude, it seemed like a good topic to review.
Optimum Altitude for Default Cruise Altitude
The Optimum altitude was designed to tailor performance predictions and provides different calculations based on the cruise speed mode selected during Performance Initialization. Most operators enter their desired (or flight planned speed), but remember, there are multiple selections that can be used for flight planning (see below).
Cruise Mode Speed Selections
Note: Not all systems have all the speed modes described below. There is some variance between OEMs.
- > For the LRC and manual cruise speeds, the optimum altitude is computed where the specific range (fuel economy) is optimized. This altitude is typically close to the ceiling altitude.
- > The MAX SPD optimum altitude is where true airspeed is maximized. This altitude tends to be close to the VMO/MMO crossover altitude.
- > For MAX END speed, the optimum altitude is where the fuel flow is minimized. For MXR SPD, the optimum altitude is where true airspeed is maximized while ensuring the destination can be reached based on fuel quantity.
If optimum altitude is selected for the initial cruise altitude on the PERF INIT page 4/5, then the computed optimum altitude is displayed as the cruise altitude on following PERF DATA pages adjacent to the Ceiling Altitude. The time, speed and fuel calculations will then be computed relative to the desired performance selection and will also consider step climbs if entered during the PERF INIT entries.
Altitude Selection and Ceiling Altitude
As a reminder, step climbs can be used to optimize aircraft performance. As the aircraft burns fuel, the optimum altitude goes up. If a step increment is entered, the FMS calculates time and fuel predictions and assumes that the step climbs will be made. If clearance is not given or the step climb is not going to be made, the step increment should be reset to zero to maintain accuracy of time and fuel predictions. Future articles will explore more performance calculations and how the FMS provides various computations.