27 November 2015, ©. Leeham Co: In an article yesterday, we looked at the economics of acquiring used aircraft for long range operations. We compare getting a second hand Boeing 777-200ER or Airbus A340-300 to operate as a long range complement to an existing network or to start a charter operation to destinations further away than previously possible.
The low fuel cost has made longer range destinations economical for a number of business models and the low capital cost and good conditions of these aircraft open new opportunities.
One of the things that must be considered in such analysis is the deterioration of the aircraft’s components. This will affect the aircraft’s performance in a marked way. In fact, an aircraft only has its factory advertised performance once in its life-time: at delivery.
Certain types of deterioration can be reset to zero by maintenance actions; others will not be reset completely by a visit to the maintenance shop. Let’s go through the different forms of deterioration that one normally caters for in performance calculations and how one go about to restore the aircraft’s operation as much as possible with scheduled maintenance.
Deterioration of an aircraft can be divided in two major parts: deterioration of the airframe and deterioration of the engines. Of these, the engine deterioration has the largest impact on aircraft performance. It manifest itself as a loss of efficiency and a gradual consumption of margins in the engine, most of them measured through the Exhaust Gas Temperature (EGT).
The EGT can be seen as a health monitor of the whole rotating part of the engine and especially the hot sections. As these get wear and tear from the bypassing gasses (erosion, deposits) and the many rotating seals in the engine gets worn, the engine becomes less efficient.
The fuel computer, the FADEC, keeps desired performance by injecting more fuel into the combustor; the engine runs hotter for the same thrust. This means that the engine is consuming more fuel during climb and cruise than it used to. At some point the prescribed maximum EGT is reached and the engine has to be taken off wing for maintenance.
The deterioration of fuel efficiency follows a rather determined pattern. For many engines, it has a first phase where the deterioration percent per thousands of cycles (wear is very much connected with take-offs and therefore to cycles rather than flight hours) is higher. Then the engine settles down to a steady lower wear rate.
The effect on the fuel consumption is profound. Typically an engine that is pulled for maintenance can have up to 5%-6% worse fuel consumption than when new. After maintenance, the engine’s fuel consumption improves but it does not go back to the factory new value. There are most times around a percent or so that cannot be regained.
The cycle then restarts until it is time for the next maintenance for the engine. Typically a short haul engine like the CFM56 makes a few shop visits for performance restoration maintenance before 20,000-25,000 cycles, when a lot of rotating parts have to be changed due to fear of fatigue. Long range engines go to shop at fewer cycles but these can have many more flight hours on wing as these are typically operated with around six to seven hours on average per flight cycle.
The airframe of an airliner is also subject to in-service deterioration. One normally calculates two sorts of wear and tear; aerodynamic deterioration and weight deterioration.
The aerodynamic deterioration comes from dirt and insects gathering on the airframe. Especially when it glues onto the leading edges of wings, tailplanes and engine nacelles, it causes an earlier transition from the low drag laminar flow to turbulent flow. This increases the parasitic drag of the airframe.
One normally also counts misalignment of aerodynamic surfaces like ailerons, stabilisers, etc., as deterioration. When they don’t sit at their factory prescribed positions during cruise, they cause extra drag that has to be counted in the flight management systems performance calculations.
The airframe deterioration normally stays within a percent or so and gets reset by heavier airframe maintenance actions like D checks or equivalent.
The same applies for weight deterioration. An aircraft in use gathers moisture in its insulation between the cabin walls and the aircraft’s skin due to the condensation that appears during decent. Modern aircraft like the 787 and A350 can be ordered with optional drier systems that circulate warm and dry air through the insulation to remove the condensation.
The weight that can be added by condensation is considerable. One calculates up to a percent of empty weight that gets added between the heavy checks that take out all insulation and dry it when the airframe is checked for wear and tear. In this figure is also included any dirt that gets left behind in the cabin and cargo pits despite regular cleaning.