September 23. 2009 03:52

I received few e-mails from the guys as to why we were flying VFR altitudes? 39,500 feet? Frankly, I am very impressed by the guy’s sharp eyes and a very valid question. I will just post the answer here for all to read instead of answering everyone individually.

In order to explain exactly why we were at 39,500 ft instead of just 39,000 first I need to explain few indications on the MCDU (provided picture).
On the top center you will see OPT indicating 395. This represent the optimum flight level, it’s the most economic flight level for a given cost index, weight, weather data (mainly the wind and ISA deviation). This information is continuously updated in flight by the computers. It ensures a 15 minute minimum cruise time and It is continuously updated in flight until 15 NM from the top of descent which It is dashed after this point. In a simple words, the computer calculate which flight level the wings will be at its most efficient point for best performance keeping in mind the compromise between fuel and time saving.

We always try to fly at our OPT flight level. 4000 ft below this level we will burn 5% more fuel, 2000 ft below we will burn an extra 1.5% and 2000 ft above the recommended OPT level we will burn 2.5% extra fuel. On most of our flight we have enough fuel to destination, alternate, and 30 mins reserve. As it cost fuel to carry fuel for any other unforeseen issues we only carry a contingency of 5% extra fuel. (for weather avoidance, hold at destination, delay departure on ground, etc..) so as you can see if we fly only 4000 ft below our OPT altitude this extra 5% of fuel which is only 10-13 mis of endurance is gone and we end up with bare minimum fuel. On this flight we were given cursing level of 39,000 ft but as the computer recommended FL395 we requested a block between FL390 and FL410 and we continuously climbed as the OPT increased. This explains the 39,500 ft.

Other info:

Top right you will see REC MAX.
This field displays the recommended maximum altitude (in magenta),
This computed based on the current gross weight and temperature.
and assuming that the anti-ice is off and it provides the aircraft with a 0.3 g buffet
margin, a minimum rate of climb at MAX climb thrust, and level flight at
MAX CRZ thrust. This field is limited to 41,500 ft on Airbus 340-500.
If two engines are out, (a very bad day) this field displays the recommended maximum engine-out altitude, which is computed based on the long-range cruise speed and assuming that anti-ice is off.

Kambiz Pourghanad (Airline Transport)