Joined: 11 Dec 2005
|Posted: Thu Feb 07, 2008 4:56 am Post subject: B737-300 Optimum Altitude
|Optimum altitude is the cruise altitude for minimum cost when operating in the ECON mode and for minimum fuel burn when in the LRC or pilot selected speed modes.
The Optimum altitude increases under the following conditions:
- In the ECON mode – when airplane weight or cost index decreases.
- In LRC or selected speed modes - when airplane weight or speed decreases.
- On each flight as weight decreases during flight.
For shorter trips, optimum altitude as defined above may not be achievable since top of descent would come before completing the climb to optimum altitude. Trip altitude further constrains optimum altitude by reducing the altitude for shorter trips until minimum cruise segment time is satisfied. For short trips, operation at trip altitude results in the minimum fuel / cost while also satisfying the minimum cruise time requirement.
Flight plans not constrained by short trip distance are typically based on conducting the cruise portion of flight within plus or minus 2000 feet of optimum altitude. Since the optimum altitude increases as fuel is consumed during the flight, it is necessary to climb to a higher cruise altitude every few hours to achieve the flight plan fuel burn. This technique, referred to as Step Climb Cruise, is typically accomplished by initially climbing 2000 feet above optimum altitude and then cruising at that flight level until 2000 feet below optimum. For most flights one or more step climbs may be required before reaching TOD.
Optimum altitude gives the minimum trip cost for a given trip length, cost index and gross weight. It provides approximately a 1.5 load factor (approx. 48 degree bank to buffet onset) or better buffet margin. As deviation from optimum cruise altitude increases, performance economy deteriorates.
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet. Projected temperature and turbulence conditions along the route of flight should be reviewed when requesting or accepting initial cruise altitude as well as subsequent step climbs.