@Planett,
And just perhaps........the representative surface is the first treated and holdover timing starts then. I would assume the representative surface must be visible from inside the aircraft in most jurisdictions, preferrably from the flight deck. (not always possible) This will be the first surface to become contaminated again when a long taxi is threatening.
Theoretically, if this surface is fine, all the others are too. A representative surface is often the wing leading edge.
@MarkMcC,
Inner wing root on the pilot's side is the preferred spot for the first application of anti-icing fluid when a two-stage decontamination is performed. Nothing aerodynamic here, just a practical operational consideration.
Often not practical to check for the flight crew because of poor visibility from within the cabin due to contaminated cabin windows (with anti-icing fluids), precipitation on the windows or darkness. Judgement from the flight deck due to the distance to the wing is often also not practical in light of the "clean aircraft concept."
A reference surface can be appointed if only one de-icing truck is involved in the operation (usually gate de-icing) but deicing on remote spots usually involves 4 trucks operating simultaneously.
That is why overhere the de-/anti-icing supervisor is in charge of the de-/ anti-icing operations and post treatment inspections. The supervisor notes at which time the first area is treated with anti-icing fluid (single step anti icing or during 2 stage de-/ anti-icing: the 2nd stage). That is when the holdover time commences and this time is reported to the flight crew together with the concentration of the anti-icing mix.
The supervisor reports when he/she has completed the post treatment inspection of the wing and other critical surfaces (some aircraft types require a tactile (hands on) inspection). Often a black stripe is painted on the wing leading edge and wing upper surface (at approx. 75% span along the wing chord) so the flight crew can check from the flight deck for contamination during the holdover time while taxying to the runway for take-off.
Green-dot