737-700 Snow
 

SLF out of Aldergrove last night. 737-700 de-iced before take off. Just prior to push back a brief snow shower came through sufficient to close runway.

20mins later on taxi a very heavy snow shower came through. Two questions went through my mind -

(1) At what point would a 2nd De-ice normally be needed? It is save to say that after engine start, heaters etc take care of ice?
(2) Is snow covering a concerns on wings/control surfaces? (I know about contaminated wings from my PPL ground school, but I assume a light covering is not an issue.

Sorry if the questions are silly. I'm just a frequent SLF and Cessna 172 driver wanting to learn.

De-Ice an Anti-Ice
 

Hello Acars,

There are two types ice protection, de-ice, (similar to the stuff you would squirt onto your car windscreen) and anti ice, which comes in various "types" and with various percentages and even temperatures which all affect "hold-over time".

Basically anti-ice gives the aircraft protection whist taxying and getting airborne, and then after that the aircraft onboard heated leading edges will then protect the aerodynamic properties of the aerofoil.

Hope this helps!

A

Holdover times
 

ACARS (now where have I come across that acronym!), It's not an exact science, unfortunately... Just to take one obvious example, your idea of "very heavy snow" might not be the same as mine, or the crew's. It's all rather subjective.

There are tables, however, giving estimates of safe "hold-over" times, for the crew's guidance. This varies according to the method of de-icing (or anti-icing) that was used, the temperature, and the subsequent type of precipitation (if any). The recommended maximum time is then added to the time the de-icing procedure was STARTED.

Wing anti-ice only heats parts of the leading edges, and is not for use on the ground. A "light covering" on the wings is an issue. Never take-off on the assumption that snow on the controls will blow away, whether you are in a light aeroplane or an A380.

If in doubt, de-ice again. But most of the passengers will not be impressed... Particularly if you are already much delayed, reaching the crew duty-hours limit, and the extra delay means you have to cancel the flight. Tricky decision, isn't it?

When there is any ice accumulating on the wings

No. On a B737-700 only part of the leading edge of the main wing is heated.

Yes.

It very much is an issue.


Most if not all airlines operate on a 'clean wing' policy. That is, no ice/snow/slush/frost/etc whatsoever allowed on the wings and tail. A bit of frost on the fuselage might be ok.

I would assume you were initially de-iced, and then another treatment took place called anti-ice.

Depending on the type of fluid used, the temperature and the conditions at the time this will give a certain 'hold over period', a time during which new built up should not occur. In addition, the flight crew will have (should have?) checked for contamination prior to take off.

If all checks are ok, you go. if not, you go back and de-ice/anti-ice again.

When I saw this thread the first thing I thought was Air Crash Investigation. Snow on the wings is a problem and the story of Air Florida Flight 90 shows this:

http://en.wikipedia.org/wiki/Air_Florida_Flight_90

there are about 4 different types of anti icing fluids, all with different hold over periods.

Now, the fluids with the longer hold over period usually have a higher viscosity compared to the shorter hold over periods.
So the rule is you can't use the longer hold over periods on small planes generally because it won't get blown off during takeoff/initial climb, this can make it have a similar effect to icing.

http://aircrafticing.grc.nasa.gov/courses.html

this is the best resource i've found related to icing, I spent about 5 hours just going through the ground de/anti-icing section. Have a read, you will learn heaps from it!

Thanks to everyone who has provided information for me. Started the course on the link provided! Probably going to scare me more than I already am!

Just for the record - I deemed it heavy snow when we departed. I ain't questioning crew judgement. I was just seeking what would lead up to a GO or NO GO decision under normal SOP.

The crew has a "hold over" chart that they go by. The de-ice/anti-ice crew will measure the specific gravity of the de-ice/anti-ice fluid. That is combined with the type of fluid used, the out side temperature and that is given to the aircrew.

The hold over chart for the specific type fluid will be broken down in graphs with outside temp and what the moisture is..sleet, freezing rain, light snow, heavy snow...etc.

Also, "de-icing" an airplane is actually a two step process...first it is de-iced, all snow/ice is removed with hot water. Then a layer of heated anit-ice fluid is applied to the aircraft to keep any moisture from re-freezing on the aircraft for a short while. Most airline GOM state that one of the pilots have to visually inspect the wings prior to take off.

De-icing an aircraft is expensive... the lowest charge I saw for my L1011 was $8,000USD and the highest was in YQX at $18,000USD.

Regards,
Greg

From the relation between visibility and snowfall intensity chart in Canada
heavy snow fall is

In darkness
-1C(30F) and above : Visibility less than 1sm
Below -1C(30F) : Visibility less than 3/4sm
In daylight
-1C(30F) and above : Visibility less than 1/2sm
Below -1C(30F) : Visibility less than 3/8sm (1/4sm for FAA)

and for the heavy snowfall and some other precipitation, there is no holdover time guideline.

http://www.tc.gc.ca/tdc/projects/air/f/tables.htm

Oh and I've sprayed 2800L type I and 700L type IV on the 737-700. It had 3ins of ice and lots of snow on top of it. It took me more than 45mins to deice it. That's about CAD$19000 on the 737 and 3 of them all by myself with no canopy:{.

And with NO holdover times in the tables for heavy snow, ice pellets etc., my reasoning is that no take-off is allowed. Certainly, an unsuccessful take-off accident board would start THERE!

GF

Boeing allow take off with "fuel frost" on the upper wings of 737-700 within a painted black line area. This is caused by landing with lots of cold fuel(c. 2 tons or more in each) in the wings which causes moist air to form frost. In freezing precipitation, however, it must be de/anti-iced, and is not allowed.

Boeing also suggest using wing anti ice on ground (which heats all but the outer leading edge devices) if engine anti-ice is used for taxi and take off in icing conditions and the plane has not been de iced with type 2 or 4 de icing fluid. Boeing define icing condtions as 10 degrees C or below and visible moisture present or vis below 1 statute mile (1600meters) or contamination on taxiways or runway.

Approx 5 yrs ago, was SLF on a DC-9 departing ORD , late Feb, abt
2300. Steady rain as we taxied out, changed to hvy snow, and the wings were snow-covered as we reached the runway. We took off without delay or deice, but we seemed to use every inch of the available concrete.

I assume airtemp was above zero, and the ground temp definitely was above freezing, as the snow was melting for the most part.

Unsure of fuel load, we were only going to MSP, but the acft was full; I was the only standby who got a ride.

In retrospect, I should have asked the cabin crew to inform the flight crew of the white wings. Or is the -9 snow tolerant?