Because of the distance which has to be covered to reach 41.000 feet which may not be convenient for a short flight.

On long flights it is more typical to fly at 41.000 feet when the aircraft weight and performance allows.

Lift is the component of the total aerodynamic force perpendicular to the relative airflow.

Lift is generated when a streamlined body, such as an aerofoil, is placed in a flow of air.

In a high bypass design, the vast majority of the thrust is derived from the ducted fan, rather than from combustion gases expanding in a nozzle.

A high bypass ratio provides a lower thrust specific fuel consumption (grams/sec fuel per unit of thrust in kN using SI units), especially at zero velocity (at takeoff) and at the cruise speed of most commercial jet aircraft.

OM = DOM + crew and fuel

A turbofan is a type of jet engine, similar to a turbojet.

It essentially consists of a ducted fan with a smaller diameter turbojet engine mounted behind it that powers the fan.

Part of the airstream from the ducted fan passes through the turbojet where it is burnt to power the fan, but the majority of the flow bypasses it, and produces most of the thrust. They are mainly used commercially because they are highly efficient and relatively quiet in operation.

Basic Empty Mass (BEM) is the mass of the aircraft with the basic equipment, unconsumable fluids and unusable fuel and oil.

Induced drag and its wing tip vortices are a direct consequence of the creation of lift by the wing.

Since the Coefficient of Lift is large when the Angle of Attack is large, induced drag is inversely proportional to the square of the speed whereas all other drag is directly proportional to the square of the speed.

The effect of this is that induced drag is relatively unimportant at high speed in the cruise and descent where it probably represents less than 10% of total drag. In the climb, it is more important representing at least 20% of total drag. At slow speeds just after take off and in the initial climb, it is of maximum importance and may produce as much as 70% of total drag.

Finally, when looking at the potential strength of wing tip vortices, all this theory on induced drag must be moderated by the effect of aircraft weight. Induced drag will always increase with aircraft weight.

They provide at least 1000ft clearance over obstacles within 25NM of the homing facility.

NDB’s are stations emitting a signal. There is nothing for them to be accurate about. ADF's on the other hand, when tuned to an NDB need to be accurate within +/- 5 degrees.

The maximum brake energy speed.

It is a function of the gross weight, altitude, temperature, runway slope, wind component, and braking configuration.

V1 must be less than VMBE.