Ice
A
small, simple word. How devastating it can be.
We
have already discussed the difference between anti-icing and de-icing on these
pages. Now let’s have a tiny peek at the effect of ice.
There
are 6.229 Imperial Gallons (7.48 US Gallons) contained in one (1) cubic foot of
water. Get your rulers out and construct a cube with each side measuring one
foot (1’).
Not
very big, is it?
That
much water, irrespective of whether it is the tiny US Gallon or the proper
sized Imperial Gallon, weighs 62.288 lb (27.76 kgs). Heavy, yes?
One
cubic foot is 1,728 cubic inches so a one-inch thick layer will cover 144
square feet (12’ x 12’)
(Sorry
about all of this math – just trying to make a point here!)
The
surface area of a small jet airliner – like a Boeing 737-200 is 6,500 square
feet (compare with your home). This is equivalent, using the above data, to
281.17 Imperial Gallons of water at one inch thick.
That
is 2811.7 lb (1253 kgs) of water. One and a quarter tonnes.
Want
to consider the top of the fuselage and the tailplane? Hmm. Thought not.
Well,
we could more than double that weight of water if we look at those other areas.
Is
a one-inch thick layer of ice outlandish – an exaggeration? Not really. This is
a perfectly feasible thickness. But, even a quarter inch thick layer of ice
over the whole aircraft will come to nearly a tonne Actually, around 785 kgs).
This is, you will recall, only a small aeroplane we are considering. We could,
if you have time, do the calculations on something a shade bigger – like a B747?
Very
well. We’ve made the point.
Ice
is very heavy.
The
enemy of aircraft is weight. The greater the mass you have the more
power/thrust you need to lift it into the air. If the total mass of aeroplane,
fuel, passengers and luggage plus the ice is too great then the aircraft will
not fly. The limit is in the books as the Maximum Permissible Take-Off Weight
(MTOW). Of course, we cannot measure the weight of the ice which is why it is
vitally important to get rid of it before take-off; it is also vitally
important to get rid of it as close to take-off as possible to prevent a
further build up if it is still snowing or there is freezing rain.
Then
there are aerodynamics.
This
hits the aeroplane in two distinct ways.
Firstly,
the ice changes the shape of the wing and also it can create rough surfaces on
the wing.
This
is critical because the wing is designed specifically to produce sufficient
lift at a certain forward speed to get 20,000lb of B737 (in our example) off
the ground safely and smoothly.
Note:
‘safely’.
Changing
the shape of the wing changes the lift characteristics of the aerofoil
(cross-sectional shape of the wing) to the point where, in extreme cases, all
lift is lost.
Certainly
there is a major move of the airflow towards stalling (the point where the work
done to lift the aircraft into the air reduces to the point where no work at
all is being done.
While
the stalling point is being approached the centre of lift – this is the point
under the wing where the air is pushing the aeroplane up, is on the move. It is
moving forwards.
Eventually
it moves so far forward that the nose of the aircraft pitches up and airspeed
reduces dramatically reducing lift even more and the aircraft now possesses the
flight characteristics of, say, a cat.
The
second consideration is that an ice build-up in the engines, especially, but
not solely, in the intake, increases the mass of the aircraft and disrupts the
flow of air into the engines.
Jet
engines really, really, like a smooth flow of air going in. Anything that
ripples or is in any way turbulent tends to disagree with the first bit of the
engine which will now stall.
This
is a bad thing.
The
engine now produces less thrust at a time when the main thing that the
aeroplane really needs is? Thrust!
The
result of not de-icing a ‘plane before take-off and not switching on the
anti-icing and de-icing systems is that the ‘Air Florida’ B737 lands in the Potomac
River after crushing vehicles on the 14th Street Bridge.
Ice.
A
small word but a big force.
