Originally Posted by
Crossunder
From the flight attendants' blog: "As they looked out the windows, they saw patches of frost along the wings"
Wow - imagine that! Patches of frost? Well, as it happens, both the B737 and A3xx, together with most other aircraft types are approved for take-off with frost on the wings. As an example, the B737 does not have any form of de- or anti-icing systems on the tail (or outer slats for the NG). This is because ice is not a problem on this aircraft.
This "clean wing concept" is mostly a political (FAA/Government) concept, and goes against what the manufacturers have found through testing. Of particular interest is the Airbus link further down in my post. Prior to 1983, Airbus allowed taking off with upper wing frost. Then the regulators told them they were wrong about their own aircraft, and the FCOM had to be changed. It has created mass hysteria, and has been blown completely out or proportions. A thin layer of frost will not cause a B737 or A320 to suddenly fall out of the sky!!! You can have three inches of rough ice on the wings and tail, make a single-engine missed approach in IMC, and the aircraft is certified for this. Come on! Whatever happened to "know your aircraft", experience and airmanship? Fortunately, the Airbus company has decided to embark on the long and arduous journey to bring back the old procedures, since tests clearly show that the FAA's rules are far too restrictive (lawyer friendly, if you will).
These aircraft and others do not deice the tail because it has been demonstrated that IN FLIGHT a build up of ice on the leading edge of the tail - which is the only place ice forms in flight on the tail - does not affect compliance with the relevant regulations. Since the regulations do not require such a demonstration for takeoff - because ice is NOT ALLOWED on the tail at takeoff - there is absolutely no way of saying "its ok in flight so takeoff is ok" - in fact, it may well be very much "not ok".
The three inches of ice you refer to is the ice shape considered for certification on the NON-DEICED/ANTI-ICED sections of the airframe. I believe most of the wing LE of most aircraft is indeed protected - and if you were to have an anti-ice system failure in icing conditions you would not be in good shape - the "failure case" ice shape is usually half the size of the 3" shape, and often its necessary to recommend speed adders and other adjustments to cater for the very detrimental effects of this ice.
The tail is in any case a red herring here, because the discussion relates to wing frost. Even a thin layer of frost on the wing, even on a large aircraft such as a 737 or 320 class aircraft,
even with slats can cause a loss of lift of 30%; that's by no stretch of the imagination trivial.
Oh, and I believe that Airbus intends to have similar rules to Boeing for the 737 once they have passed the regulatory hurdles, that is, "non environmental" frost only. Which means, in practice, cold fuel frost on a warm day would be ok, but winter frost - such as seems to be the case in the incident in question - would still be forbidden. (See slide #23 of the presentation you linked to)