"In thrust, we trust!"
|Hazard warning decals on a CFM56 engine nacelle|
Speaking of turbofans engines, those are the ones with transonic velocity regime (Mach numbers from 0.75 to 0.9). Looking into the front core of the engine, the large fan can be seen, so the air entering the core first passes through the fan and is partially compressed by it. Most of the air, however, bypasses the core and goes directly to an exhaust nozzle. This is why these engines are also called as bypass engines. The remaining air inside, called the core stream, proceeds directly to its own exhaust nozzle, through a series of compressor blades (high- and low-pressure), to the combustion chamber and leavers the nozzle after the turbine stage.
|Schematic illustration of major components on an engine (Trent XWB engine on Airbus A350-941, F-WWCF)|
Another major issue is the engine inlet and its risk of ingestion. The working principle of jet engines is based on simple physics. An operating engine will introduce low air pressure in the inlet. As a result of this low pressure, the air will move towards the engine core. As the air flows into the engine, the amount of air near the inlet will have higher velocity that the rest, further away from it. The suction is the strongest at the inlet. (Think about a similar example in a bathtub full of water. If you unplug the tub, the water will start flowing out. At the outlet, which could be our engine inlet, huge suction is present.) Due to this huge suction effect of an engine, it is dangerous for any ground personnel to stay in the vicinity of a jet engine.
The clearance distance for high-bypass engines, with 2-3 meters fan diameter, is usually about 10 m to the front and 50 m to the rare. The picture shows the hazard areas for CFM56 engine, which has fan diameter of only 1.55 meter. This area however can vary with different engine power settings.
The following collection of videos are intended to show how dangerous the blast, or exhaust of a running jet engine can be.
Starting with a van that is being destroyed by the jet blast:
Following with a famous car vs. Boeing 747 example in Top Gear TV series:
Lastly, humans can be blown away too, which occurs usually at St. Maarten:
Another topic involved with jet engines is: F.O.D. (Foreign Object Debris/Detection/Damage), which now should be obvious why it is dangerous for the engines and rotating elements. Foreign object damage is a result of any foreign debris or object that is sucked by the inflow. The debris can come from any ground vehicle, wind, ground personnel or even from other aircraft. The importance of a clear ground and runways is therefore crucial at areas where airplanes are taxiing and flying. Not only it is a threat on the ground, as flying birds can be sucked too - known as 'bird strike' incidents. Coming back to the damage that they can cause are very varying. Engines are designed to withstand major damage that could be caused by small metal objects. Special tests have to be passed in order to give certificates to any engine. Usually a frozen chicken is shot to the engine, which has to keep all of its damaged parts inside the nacelles. They can not cause any further damage to the rest of the aircraft. The following video is presenting this testing.
The major issue here is however the functionality of the engine later on. The foreign objects could break off blades, those metal parts will cause further damage to other parts, which all will result in a reduction of thrust, or even an engine shut-down. This is very crucial when approaching the ground or during takeoff. Bird feather and bones are the most unwanted "by-products" of a bird strike, which can seriously damage the turbine blades by clogging its cooling holes.
I would like to finish with a funny but at the same time memorable example of a FOD case. It happened to a Delta Airline's plane, when during taxiing, another aircraft's jet blast blew the empty cargo container away, which was then sucked by the inlet of another taxiing airplane. All the topics of this article is covered on one photo.
|Delta Air Lines cargo container in engine, Febuary 1999|
Have you noticed too that the warning decals are missing from the side of the engine?
Video's URL, in order of appearance:
- http://www.boeing.com/commercial/aeromagazine: Fred Zimmer - Preventing Engine Ingestion Injuries When Working Near Airplanes
Video's URL, in order of appearance: