Debrief: MET Towers: A Collision Can Happen and it Has Happened...

by Eduard Alf, P.Eng., Visual Aids Technical Unit, Aerodromes and Air Navigation Division, Standards Branch, Civil Aviation, Transport Canada

The spraying of crops by means of a specially adapted aircraft is a common activity in rural areas. In order to obtain the most effective application, the aircraft is often flown at heights in the order of three to four meters off the ground. The field, however, may also have a meteorological (MET) tower, which is used to gather data for analysis of the wind resource prior to the construction of a windfarm. These towers have a tubular steel mast that is held in position by sets of guy wires.

Drawing of a MET tower

MET towers are not normally at a height or location near an aerodrome or recognized flight route, which would require them to be either marked or lit, as stipulated in Transport Canada CAR 621.19. For the same reason, they would not be identified on navigational charts.

Both the mast and guy wires of a MET tower may be quite difficult to see, depending on the ambient lighting conditions and direction of approach. The photo below illustrates this potential problem well.

Photograph of a MET tower in the distance

The June 2010 occurrence

On June 29, 2010, an Air Tractor 502B was engaged in aerial application near Portage la Prairie, Man., when it collided with an unmarked metal wind power test pole approximately 56 m high. The pilot elected to perform a precautionary landing in a nearby field. Inspection of the aircraft revealed damage to the propeller, right landing gear, flap and wing leading edge, approximately 1.2 m from the fuselage.

Photo of the damage to the wing leading edge

The photo of the damage to the leading edge clearly shows how fortunate this pilot was in terms of where the aircraft struck the pole. Had the aircraft hit the pole further out on the leading edge, aircraft control may have been lost. According to the operator, the structural integrity of the Air Tractor wing next to the fuselage is believed to have allowed the aircraft to remain airworthy and retain controllability. The top of the pole was damaged and a galvanized guy wire 3/8 in. thick was severed. The Transportation Safety Board of Canada issued a Class 5 report (A10C0101) on this occurrence.

Prior to doing an aerial spraying, the pilot or operator should always contact the field owner directly to find out if there are any objects of concern in the field. If such a tower cannot be readily seen under certain conditions, there is a good chance it will not be detected by an air reconnaissance alone. The pilot or operator should also ask the field owner if there are any MET towers in adjacent fields, over which the spray aircraft might make necessary turns.

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