Bjorn’s Corner: Aircraft engines in operation, Part 6

By Bjorn Fehrm

February 24, 2017, ©. Leeham Co: After having analyzed how the engine gets stressed during different phases of flight, we now look into how engines are used. The de-rating of engines for takeoff is important, as not 100% thrust is needed for all takeoffs. If the aircraft is lightly loaded or is taking off from a long runway, with low temperatures or altitude, the engine can be thrust de-rated so that it experiences less stress.

Once in the air, the engine is run below maximum settings by use of cost-index. These actions will result in less fuel usage and also longer engine operation between overhauls. We will now finish the operations part of our engine clinic with how airlines keep the engines away from the workshops by swapping the engines between fleet aircraft.

Figure 1. Principal picture of a direct drive turbofan. Source: GasTurb.

A visit to the engine workshop costs in the millions of dollars, so the longer the engine can operate before a shop visit, the better.

Engine families

We described in previous Corners that the engines are produced as a base engine and then rated for use on different aircraft variants. In our example, we look at a CFM56-5B engine for the Airbus A320 series.

The CFM56-5B was developed from the CFM56-5A when the initial variant could not be stretched to cover the A321. The highest rated CFM56-5A stopped at 25,000lbf thrust and the A321 needed north of 30,000lbf.

The CFM56-5B was the result, with 32,000lbf takeoff thrust for the highest rated variant (-5B3). In total, nine different variants were defined between 21,600lbf (for the A318) over the 27,000lbf (-5B4/7 for A320) to the -5B3. The engines can be configured for different thrust ratings by exchange of a thrust plug in the engine control computer (FADEC).

A higher rated variant will cost more to purchase and run into a maximum EGT maintenance action quicker than a lower rated engine.

The maximum allowed EGT is the same between the engines. Lower rated engines have a lower indicated EGT at, for example, takeoff thrust, as less fuel needs to be injected into the combustor to achieve the lower thrust level.

Swapping engines

Airlines with several types of aircraft can use the different ratings to exchange engines between aircraft variants. An engine can be purchased for an A321 as a -5B3 and operated on that aircraft until the maximum EGT limit is reached.

We saw in the last Corner that the engine starts with a 66°C margin to maximum EGT. Let’s say the engine is operated until 10°C remains, then is taken off and mounted on an A320.

This aircraft needs a rate plug of 27,000lbf. This lowers the EGT with (from last Corner) 109-66=43°C. Now we have 53°C margin to maximum EGT. This margin enables the engine to run for another 5,000 to 7,000 flights on the A320.

Should there be an A319 in the fleet, the rating required would be 23,500lbf and we would have an additional 45°C margin. Justifying another swap will depend on many factors, but it will certainly be considered.

As described in the last Corner, the gradual EGT rise due to engine deterioration is the main cause for taking engines off wing before any hard limits are reached (fatigue based LLP limits). Engine swaps to lower thrust installations are therefore an attractive option for the airlines to keep engines on wing for the maximum time.