Why climb dominates the fuel budget
An aircraft's fuel burn across a flight divides into distinct phases: takeoff, climb, cruise, descent, and approach. Climb is by far the most fuel-intensive phase relative to distance covered because the aircraft is simultaneously accelerating to cruise speed and gaining altitude against gravity. During a typical climb from sea level to cruise altitude (25,000-35,000 feet), an aircraft burns a substantial fraction of fuel while covering only a small distance on the ground.
A two-hour flight covering 900 miles might allocate 15-20% of total fuel burn to the first 20 minutes of climb, where the aircraft covers only about 80 miles. The remaining 820 miles in cruise consumes roughly 60-65% of fuel at a far lower burn rate per mile. This disparity is why long flights are more efficient per mile than short flights: the climb overhead is spread across more cruise distance.
Cruise efficiency and the descent advantage
Cruise flight at altitude is the most fuel-efficient phase because the aircraft has reached optimal speed and altitude for the engine cycle, and thrust needs only balance drag. Airlines optimize this by climbing to altitude as quickly as practical, cruising at optimized fuel-burn speed (often just below the speed of maximum range), and descending gradually when possible rather than maintaining altitude longer.
The descent phase typically burns only 3-5% of total trip fuel because engines operate at idle thrust and gravity assists the aircraft downward. This is why airlines talk about "top of descent" points calculated from cruising altitude: starting descent too early wastes fuel through unnecessary long descent time, while starting too late either forces a steeper descent (increasing drag and fuel burn) or risks arriving below the intended landing altitude. Modern flight planning systems calculate these fuel-optimal profiles to minimize costs.