Jet engines spinning in the wind.

[Parvaz]

Recently I was sitting in the jet, enjoying the weather and a few
minutes of “down time” between flights that was parked on an airport
ramp. The sun was shining, temperatures in the low 70s (mid 20s C)
and a steady wind was blowing at about 20 mph. At this particular
airport the passengers still walk outside on the ramp from the
terminal to the jet and climb up the portable stairways into the
cabin. The airplane happened to be positioned in a manner that
allowed the wind to blow through the engines. With more than 20 mph
of steady wind blowing through the engines the fan sections were
spinning happily making their typical clattering noise.
One of our boarding passengers made a malicious comment to a flight
attendant about the engines (he had to walk by one of them on his way
to the jet). Raising a safety concern or asking a question is in no
way malicious on its own and should be encouraged. But just like
every human interaction body language, facial expressions, tone and
inflection have a significant influence what is communicated. In his
opinion there was something wrong with the engines. His comment
vaguely implied that something needed to be done to “fix” the engine
before we tried to go flying and that we were knowingly jeopardizing
his safety. That unfortunate exchange provided the background for
this discussion. Interestingly enough after he made his comment he
proceeded to walk back into the cabin. Finding his assigned seat he
sat down-apparently not really too concerned about the safety of the
engine he had just raised an issue about. To assuage his concerns I
included a reassurance during my routine welcome aboard passenger
announcement that the rotation of the fan section and the accompanying
clattering noise was completely normal given the windy conditions
outside.
If you spend a lot of time flying it is helpful to have some basic
understanding of jet engines. Unlike your automobile engine the jet
engine does not have pistons or crankshafts. Today’s commercial jet
engines are typically “fan jets” also known as “high-bypass” jet
engines. There is a large multi bladed fan attached to the jet
engine’s front end, at the back end there are several turbine wheels
mounted. The whole assembly front and back, is connected and rotates
freely-just like your decorative back yard ornament spins and twists
in the afternoon breezes. Normally there are several concentric
spools involved: turbines at the back end, rotors and a fan attached
at the front end. Those two spools rotate independently of each
other. One of our external preflight duties includes grabbing the fan
section and giving it a good spin to induce rotation. We then look
for smoothness, symmetry, and freedom of movement while listening for
any abnormal sounds.
Thrust is produced when jet fuel is introduced into extremely high
pressure air and ignited. The tremendous release of energy generated
from the burning of the rapidly expanding air fuel mixture is directed
onto the turbine wheels attached to the back end of the jet engine
causing them to rotate. That rotation is mechanically transferred
through those concentric spools causing the fan (and rotor section) to
turn. The rotating fan gulps in vast quantities of air clawing its
way through the atmosphere, some of that air is routed internally and
burned. Surprisingly the majority of the thrust generated in
commercial jet engines comes from the fan and “bypass” air rather than
from the air that is routed through the interior of the engine and
burnt. There are other types of jet engines namely ramjet, scramjet,
and “pure” jet engines but they are not currently used in commercial
airline service.
It is perfectly normal to see the fan section rotating if there
happens to be a strong wind blowing through the engine. It takes the
fan section some time to stop spinning after the engine is turned off
due to momentum. Consequently it is common to see the fans continue
to rotate for a short time after the jet has arrived at the gate and
the crew has shut down the engines. I have several pictures of the
fan section in a jet engine uploaded in the “Files” section of this
blog.