Aeroengine is an engineering marvel. A typical engine on a commercial airliner contains a number of fan blades, discs, turbines etc rotating at high speed alongwith other intermeshed moving metal parts. Notwithstanding the complexity, the design & technology as well as a robust maintenance schedule ensures that aircraft engines run with absolute reliability until end of their life cycle totaling thousands of hours. Although rare, but an aeroengine can fail or may require to be shut down in flight for variety of reasons including bird hit, volcanic ash, icing at engine inlet or unusual aircraft attitude etc – leading to a situation called In-Flight Shut Down (IFSD) of engine.
Despite IFSD of engine, aeroplanes remain safe to fly. Thankfully, aircraft designers have ensured that aeroplanes are capable of being flown to safety with the residual power from the remaining engine(s). This can be best understood with this example – a twin engine long hauler like a Boeing 787 which flies across oceans is certified to fly for SIX hours after shut down (IFSD) of one of its engine!!! That apart, pilots are often able to restart a failed engine and continue to a safe landing as soon as possible.
Failure or inflight shutdown (IFSD) of one of the engines is a well practiced drill by the commercial airline pilots every six months in a simulator. Pilots assume worst case senarios in simulators and fly the plane manually to land them with single engine. Infact, the emergency becomes easier to handle when the autopilot of modern airliners like Airbus 320 automatically make necessary adjustments in power and to counter directional control due to the loss of engine power. Often, the autopilot is able to fly the plane without pilot’s intervention. If IFSD is the only emergency on board, be assured that the crew would handle it seamlessly everytime.
However, Southwest Flight flight SW1380 from New York to Dallas on 18 Apr 2018 was a different story – It had suffered an “Uncontained / Catastrophic engine failure” with consequential emergencies. SW 1380, operated with a twin engine Boeing 737 – 700 with 148 souls on board was at 32000 feet when the breakaway parts of the damaged engine did not remain “contained” within the engine as expected. They flung out and hit a passenger window leaving a gaping hole in the cabin through which one passenger was almost sucked out. The gap in the window also resulted in rapid depressurisation of the cabin followed by the activation of the oxygen masks in the cabin. The cockpit crew also donned their oxygen mask, run their checklists, initiated an emergency descend and diverted to Phillidelphia where a safe landing was executed. In the cabin, the crew was handling the traumatised and injured passengers on board. Until landing, the pilots was not aware that shrapnels had also damaged secondary flight controls like leading edge flaps and slat. Evidently, the crew was faced with multiple emergencies and uncertainties which was handled professionally.
Having said that SW 1380 suffered an “Uncontained / Catastrophic” engine failure”, let us try to understand this terminology as opposed to the so called “contained” engine failures. Actually the body of an engine (called casing) is designed so strong that, if any rotating part like the fan blades breaks away, the snapped parts will not be able to pierce the casing. This will ensure that the disintegrated parts remain within the engine body. If at all, it will exit straight out of the engine’s exhaust without causing any damage to the wings, fuselage, control surfaces or other areas of the plane. This is called a “contained Engine failure”. If the broken parts fly out, as in the case of uncontained engine failure in SW 1380, they can become shrapnel or projectile that can rupture fuel tanks, pierce the fuselage and injure occupants or cause damage to control surfaces thereby jeopardising the safety of the flight.
Engine manufacturers demonstrate ability of their engines to “contain” engines failures due to disintegrated fan blades (see link below). This is invariably a prerequisite before they receive certification of regulatory agency. Despite all this, the SW 1380 incident has brought to the fore similar incidents earlier on ground and in air.
Investigating agencies including stake holders like engine manufacturers have a mammoth task ahead as the incident aeroengine model (CFM 56) powers the widely used Boeing 737 NG fleet. It is therefore to be seen by the investigating agency, NTSB as to how the failure went “uncontained” in this case. Although the immediate focus would be to determine in the first place as to why a fan blade of the engine broke away and if the underlying reason can manifest in its engines installed on the B 737 worldwide and other fleets.
Let us wait for NTSB to come out with the actual cause of the accident.
Happy landings !!
How an turbofan aero engine works – https://www.youtube.com/watch?v=_LaKlE2h3Jw
Wiki Explains Uncontained Engine failure – https://en.wikipedia.org/wiki/Turbine_engine_failure#Notable_uncontained_engine_failure_incidents )
Certification of Engines for Containing debris – ( https://www.youtube.com/watch?v=736O4Hz4Nk4 )