Tuesday, May 1, 2012



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.


A small word but a big force.