JAA Question on Optimum Altitude

[Greg2041]

Okay, this JAA question was asked in 2001 on PPRune but no-one seemed to understand the question or terminology. I have been assured that answer A) is correct so in light of a further 7 years passing, are there any takers to explain?

For a long-range four-jet aircraft in cruising flight, the optimum altitude and the lock-on altitude increase. The most cost-effective flight plan will consist of choosing cruising levels which increase during the flight in order to fly:

A) about the optimum altitude
B) about the lock-on altitude
C) between the lock-on altitude and the optimum altitude
D) just below the optimum altitude

[coodem]

Optimum altitude is the cheapest altitude to fly for a given mass/speed. So it makes sense to fly as close to it as possible. Its all about money. About means below or above, Most would be above,then as fuel is burn't the mass decreases, so slowly it would get to the optimum, and after some more time it would be below it. Then they would probably ask to climb above the optimum altitude again

[flightlevel1985]

Id say its a, about the optimum altitude as it's all about the fuel/speed/altitude balance to get the maximum range at the lowest costs, which as fuel mass decreases, A/c climbs higher to keep the balance

[Greg2041]

Very helpful. Anyone know what "lock-on altitude" is?