- Issues include engine performance limitations and the need for technical stops to refuel.
- The exercise of second freedom rights allows airlines to refuel or perform maintenance in a foreign country.
- Engine performance at high-altitude airports is limited, requiring more fuel and a longer runway for takeoff.
Commercial airlines that offer ultra-long-haul flights are limited by the operating range of the aircraft they deploy. Depending on the size of the aircraft, it can only fly a certain distance while carrying a certain amount of load (passengers and cargo). When the range of the aircraft does not meet the desired travel distance, the aircraft must perform a technical stop for refueling. Airports that are situated at higher elevations add an additional performance constraint to the already tricky equation.
Engine performance (particularly during takeoff) is considerably limited at high-altitude airports. As such, the runway length, fuel planning, and maximum takeoff weight (MTOW) are crucial for operation. Long-haul routes operated from high-altitude airports and including technical stops must be carefully planned to ensure the route is not only possible but economically viable for the airline.
Second Freedom flights
The International Civil Aviation Organization (ICAO) describes the exercise of the second freedom rights as refueling or carrying out maintenance in a foreign country without embarking or disembarking passengers or cargo. For example, a flight from Sydney, Australia, to Dubai, flown by Qantas, refuels at an airport in Singapore.
While no passengers or cargo is loaded or offloaded in Singapore, the airline only considers the viability of the stop in terms of airport fees, route scheduling, and the amount of fuel required to get to Singapore. The equation becomes a little more complex when an airline operates a second freedom flight from a high-altitude airport.
Let us consider an Ethiopian Airlines flight from Addis Ababa (ADD) to Washington, DC (IAD), with a fuel stop in Rome, Italy (FCO). Flying out of ADD, located at an elevation of 7,550 ft (2,300 m), means limited engine performance. As the air density changes at higher elevations, the density altitude must be optimized to offset the reduced performance of the engine.
Engine performance is also limited by the maximum temperature that the engine turbines can withstand. As such, engines rated at a specific thrust at sea level would not generate the same amount of thrust at high-altitude airports. With a reduced engine performance, the engines require a greater amount of fuel and a longer runway for takeoff.
Photo: IanC66 | Shuterstock
High-altitude conditions may also impact the amount of load the aircraft can carry. These parameters also influence the location (in a third country) selected. Aircraft performance tradeoffs must also be calculated while considering the distance to the fuel stop.
For example, Ethiopian Airlines recently swapped its Dublin (DUB) fuel stop with Rome (FCO) between ADD and IAD. Considering all the aircraft performance parameters, the overall route must also be economically viable for the airline. The flight distance for ADD-DUB-IAD is 7,360 miles (11,845 km), whereas on the ADD-FCO-IAD route is 7,280 miles (11,715 km). The difference may be marginal, but striking a long-term economic deal with the fuel-stop airport while optimizing route planning can make a substantial difference to the bottom line.
What are your thoughts on optimizing parameters when high-altitude airports are used for long-haul routes? Have you taken a flight with a fuel stop? Share your experience in the comments section.