By Bjorn Fehrm
March 14, 2018, ©. Leeham Co: India’s Directorate General of Civil Aviation (DGCA) grounded Airbus A320neos equipped with Pratt & Whitney GTF engines with faulty compressor seals Monday.
Affected are eight A320neos of Indigo airlines and three A320neos of GoAir. The Indian groundings are unusual as they go beyond the directives of EASA and FAA for the problem.
The Pratt & Whitney GTF engines for the A320neo have had a troubled start to operations. While the engine’s performance is better than specification, the reliability is plagued with teething problems.
The first operational problem was long engine start times, partly to blame on Airbus method of hanging the engine by their midpoint (the core). All Turbofan engines experience the bending of their rotating parts when being shut down. It’s called rotor bow and is caused by the standing heat gathering at the top of a non-moving rotor. The A320 midpoint mounting augmented the rotor bow of the GTF.
The cure is to vent the engine by cranking it before the start procedure, by it giving the rotor a more even temperature before spinning it up to operational RPMs. Engines hung at the fan case and tail (Bombardier CSeries, Embraer E2 and Mitsubishi MRJ) do not have prolonged engine start times. The fix for the A320neo was changes to the engine and to allow both engines to be ventilated at the same time, by it shortening the total engine start time.
Next problem was engine oil metal detector warnings. The third engine bearing compartment had a seal which didn’t work correctly at high Flight Levels. The thin air caused the air riding seal members to sometimes oscillate and touch. This left metal particles in the engine oil, which caused warnings pointing to a potential engine problem. The fix was a change to a more classical carbon seal.
The third problem was combustion chamber linings which had unforeseen hot spots, reducing the linings operational life. The fix were liners with modified cooling.
These problems were addressed and the fixes were built into new engines when a more serious problem occurred over the last six weeks.
The above problems are all of a non-serious character. They cause operational problems for the airlines, but they have not caused a flight hazard. Engines are worn prematurely and the bearing seal problem gives warnings (metal chip detection in the oil) which must be treated as if it was a serious problem.
Over the last month, a more serious problem has occured. And it was unexpected. A high-pressure compressor seal of the standard knife-edge type suddenly cracks, pieces of the seal breaks loose and enters the compressor, combustor and turbines. The seal is a modified version of a previous knife-edge seal in the area.
Knife-edge seals have been used in gas turbine engines since the 1940s. It’s a standard method to seal a higher-pressure compressor/turbine stage from a lower pressure stage, Figure 1.
Why is the improved seal (the first version showed to high wear on its seal surfaces) cracking within months of starting its life? We don’t know, but premature metal fatigue could be a root cause. How can a seal fatigue in such a short time? If it enters a self-induced oscillation for some reason.
Seals are the barriers between high-pressure areas and the leaking air over the seal can induce an oscillation and such oscillations can quickly cause metal fatigue in the seal. This is pure speculation, but it has happened before in other engines.
The real problem is, it wasn’t found in testing before production. We will know why when Pratt & Whitney and EASA/FAA tell us what happened. Seal parts that come loose means the engine downstream of the seal gets damaged and loses thrust. The pilot action when this happens is to put the engine to idle and interrupt the flight.
The short-term fix is straightforward. Re-introduce the original seal; it doesn’t crack and cause a loss of thrust. This is done for engines produced from now. For affected engines, the faulty seals will be changed. The operation should be finished by June.
Then over time, develop a longer lasting seal which is thoroughly tested before being released for production.
Affected engines are serial number 540 and later. Pratt & Whitney has produces 100 engines with the updated seal. EASA and FAA have issued an Airworthiness directive that no A320neo shall fly with two affected engines. It’s OK to fly when one late series engine is mixed with one which doesn’t have the troublesome seal. Extended operation over water is not allowed with A320neos with affected engines.
The decision of India’s DGCA to ground all A320neo with an affected engine is strange. Flights with mixed engines are not allowed. Normally the operation country authorities follow the authorities in the countries of the airframe and engine origin. Not in this case. Perhaps the close timespan between incidents in India (three since 29th of January and the last Monday this week) can explain the grounding.