There are basically 2 altitudes calculated for any aircraft type, usually by the FMC (Flight Management Computer). One is maximum altitude, and the other is optimum altitude. These altitudes can also be extracted manually from performance charts by the pilots.
Let us first look at maximum altitude. This is limited by
- aircraft weight
- outside air temperature
Generally the lighter an aircraft is, the higher it can fly, however for each aircraft type there is a maximum certified altitude (41,000 feet in the case of the B737-800, 39,000 feet in the A-320, and 37,000 feet in the B737-400). As air temperature increases above standard (standard is 15 degrees C at sea level, decreasing to -57 degrees C at 36,000 feet, the level of the tropopause), the density of the air decreases, and hence it is less able to provide lift.
Optimum altitude is limited by
- maximum altitude (obviously you cannot fly above your performance-limiting altitude)
- maximum certified altitude
- trip distance (see below)
- aircraft weight
- economy (or fuel cost)
To explain this in a little more detail, let us consider 2 aircraft of an identical type, both about to depart London. To keep it in perspective, let’s say they are both B737-800’s, and each is carrying 150 passengers.
The first aircraft is bound for Manchester, distance of just over 130 nautical miles, whereas the second is bound for Cairo, almost 2000 miles distant.
In the case of the Manchester flight, it will only carry enough fuel to fly to Manchester, divert to an alternate, and a certain fixed fuel reserve. Carrying fuel in addition to these requirements would entail additional cost (increased fuel burn), so unlike in a car, aircraft determine a specific fuel load for each trip. Thus the takeoff weight for this aircraft might only be 62 tonnes.
The 737-800 can climb to 39,000 feet at this weight, however the time and fuel burn required to reach 39,000 feet means that it would probably fly all the way to Manchester before reaching this altitude. Hence the FMC would calculate an optimum altitude for this comparatively short flight, and it would be in the order of perhaps 28,000 feet.
The Cairo flight on the other hand would be more likely to have a takeoff weight of 75 tonnes, due to the greater fuel load required, which would limit it to a maximum of around 35,000 feet. As the aircraft burns fuel, and thus reduces weight, it could climb higher, so about 2 hours after takeoff it could climb to 37,000 feet, and after another couple of hours to 39,000 feet. Thus optimum altitude varies as a function of aircraft weight. In order to achieve maximum efficiency, aircraft are best flown at, or as closely as possible to optimum altitude.
Another couple of variables; as our Cairo flight flies into warmer air (I.e. less dense air), optimum altitude may decrease. In practice, this may mean that having already climbed to say 37,000 feet, the flight may briefly be above optimum altitude when it flies into warmer air, however this would be only a very temporary situation, given that optimum altitude is constantly increasing as fuel is burned.
Another variable affecting optimum altitude can be wind. Whilst the benefits of a tailwind are obvious, it can sometimes be prudent to fly at a lower altitude, consuming more fuel per hour, than to climb up into a stronger headwind. This is because the time that the aircraft is in the air (and thus exposed to the headwind) is reduced at lower altitude.
I hope this helps to explain a little about optimum altitude. If my explanation seems long and complicated, you will understand why it is much easier to extract these figures from an FMC than to calculate them manually!
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