The Ice Regulators Cometh

Air Safety Week, March 5, 2007

The NTSB Examines The Other Side of Ice

The NTSB has again addressed the other side of icing -- airborne accumulation.

In last week's Air Safety Week (Feb. 26 issue) we addressed the fatal consequences of attempting takeoff with an upper wing surface roughened by hoar frost - see "At the Leading Edge of Aviation". In that article we advocated leading-edge flaps (generally known as slats) as one means of avoiding the "airborne into the stall" rotation - and we referred to a number of exemplar accidents.

Bombardier, for one, has apparently come around to that way of thinking in its latest series of offerings of jets with full-span leading-edge slats (see image at tinyurl.com/2lk3xq)

Jets with swept wings and clean leading edges are particularly prone to rotating into the stall if there's any roughness at all on that upper wing surface. There've been two such accidents in the space of three weeks.

FAA rules say that no frost on the upper wing surface is allowed and only 1/8th inch on the lower surface (under the fuel tanks). If there is frost, snow or ice deposits on the upper wing surface, you must de-ice before takeoff, and take notice of Holdover Times according to the level of snow falling or blowing in the wind.

You are allowed three mms of frost under the wing where the fuel tanks are, simply because there's no airflow separation risk/stagnation point on that wing leading edge undersurface. The airplane itself can have thin Hoar Frost all over the fuselage as long as lettering and surface detail can be seen translucently. It is quite valid to put warm fuel into the wing tanks to melt the ice on the upper wing surface. It will remain wet, but it will not refreeze on the ground for a long time. Once you get airborne, it doesn't matter- you will quickly get up to high speed- remember you can take off into supercooled rain. All that matters is getting airborne and the first minute of flight.

As long as you start your takeoff roll with no frost, ice or snow deposits on the upper surface of the wing, you are "in aerody country". See tinyurl.com/7leqz as well as JAR-OPS 1.345, particularly AMJ 1.345. The NTSB alert on apparently innocuous ground icing is at tinyurl.com/2w9uvx. It has become such a critical and recurring area of concern that it's now a science:

From Boeing's 'Cold Soak' publication, May 2004:

Boeing 737's will incorporate CSFF permissible area (Cold Soaked Fuel Frost) in production starting with line number 1538 in July 2004, as a standard feature with:

- Lines on wing upper surface and

- Flight Manual updates to support operations will include Airplane Flight Manual, Airplane Maintenance Manual, and Flight Crew Operations Manual

Retrofit will be available as per Boeing Service Bulletin 737-11-1125 (see Boeing's illustrated PowerPoint at tinyurl.com/36p47r ).

NTSB has now issued, on Feb. 27, its Safety Recommendations stemming from a 2005 final approach icing accident (the Feb. 16, 2005 loss of a Cessna Citation near Pueblo, CO). Six Circuit City Stores employees and two pilots were lost in the accident.

The Board determined the accident was caused by the flight crew's failure to effectively monitor and maintain airspeed, and comply with procedures for deice boot activation on their approach. This led to a sudden aerodynamic stall. Another company C560, flying in consort, did all the right things and suffered no grief.

Contributing to the accident, once the crew was under threat, was the FAA's failure to establish adequate certification requirements for flight into icing conditions, which led to the inadequate stall warning margin provided by the airplane's stall warning system. Essentially, because of the "upped" stall speed due to the wing's icing accumulation (aerodynamic deformations and weight), there was no warning.

Along with the usual admonitions emphasizing "additional training" on the Cessna 560's deicing systems, and the aircraft's flight characteristics in icing conditions, the Board also recommended the FAA require proper training on all aircraft equipped with pneumatic deicing boots that are certified for flight into known ice (FIKI).

A recommendation that the FAA mandate all booted FIKI-certified aircraft to be equipped with automatic cycling systems, to insure the boots continue to cycle once activated; well that's a sure-fire life-saver.

The NTSB also reiterated revised icing criteria, a recommendation first made by it in 1996 -- to include such factors as liquid water content, and the size of ice and/or waterdrops on a plane's surfaces -- and that freezing rain and mixed ice conditions also be added to the Part 25 Appendix C icing certification envelope. In shorthand, herewith are some of the Board's icing specific points:

* Training to emphasize that pilots increase airspeed and operate the deice boots during approaches when ice is present on the wings.

* Emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions.