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UNCTUOUS
8th Nov 2003, 00:01
Re the ATR-72 freighter downing by icing in the Taiwan Straits (ASC interim report on TransAsia Flt GE791 recently appeared on their web-site)

Can any ATR-72 rated pilots validate these concepts? (in particular the one at A concerning differential port & stbd wing ice accretion)

As to what finally induced the “loss of control” roll after the autopilot ran out of ideas, you are once again spoilt for choice:
1. the aircraft wouldn't cope with an air intake blockage or ice breaking off type ingestion failure of an engine (without instantly departing controlled flight)
2. Ice-contaminated tailplane stall (ICTS)
3. Some autopilots can also hold (as in hide) large rolling moments (as against auto pitch-trim disguising large changes in longitudinal weight distribution and drag effects) - these may be due to ice-choked engine inlets and low torque on one side.but it’s more likely to do with asymmetric wing icing…..


A Re the ATR-72 Icing Stall

Probably an important additional point to make (and possibly one that hasn't occurred to Kay Yong of the ASC), but the reason why turboprop asymmetric stalls occur (invariably as instant autorotations or spin entries in the same direction) in severe icing conditions is only apparent when you think about the wing (and perhaps tail) ice formations inboard being differently affected by the propeller rotation direction (and exhaust configuration) being the same on both wings' engines.

On the port wing the effect of prop rotation direction would be to increase the wing ice build-ups inboard of the engine nacelles (and on the starboard wing, the exact opposite, i.e. outboard of the nacelle). So, as the ice builds and the stalling speed increases (because of aerodynamic efficiency losses) there is also the hidden danger of the two wings now having drastically different coefficients of both drag and lift (and importantly, in the spanwise distribution of lift). Port wing stalling angle 15.3º AoA & Starboard wing stalling angle 17.1ºAoA =a guaranteed result of a wild roll to port at 15.3 AoA

B. Turbo prop Icing Solutions

A new emergency transponder code: The standard 7700 emergency squawk reverting after a minute to 7400 would immediately tell ATC the aircraft is iced up and about to enter an emergency descent and all aircraft at levels below should be cleared. Reluctance to act can mean the precipitation gets the icy upper hand.

Slippery stuff: A waxy/resinous coating might be employed as a wintry semi-permanent inflight de-icer. This Teflon-like non-stick surface coating would be sprayed seasonally on upper surfaces and leading edges of the wings, nacelles and tailplane. Adding little weight or drag, the coating would limit the amount of ice that could build up (before departing due to lack of stiction). Add a color marker. The colored wax would be seen to be present, and a significant loss of color would indicate an ice buildup beyond the capability of the debonding wax agent – thereby predicating an urgent descent escape. A new range of environmentally friendly methyl carbitol-based range of waxes, resins and film depositions may be suitable. It should be possible to find a suitably slippery coating that can stop lethal buildups of SCDD ice over the whole airframe.SCDD = super-cooled drizzle droplets.
It should be noted that this concept has been seriously explored, although not with some of the new coatings alluded to above. The paradox of Teflon is that when supercooled liquid water strikes, the liquid forms ice in the “pores” of the Teflon. By far the biggest problem is that coatings analyzed to date can erode in the rain. The use of coatings has been encouraged by the FAA but are not allowed for certification purposes, as an effective means of determining if the eroded coatings are still effective, and for how long.

Thermal laser wiping: The theory is that one laser head sits atop the cockpit (and one under the nose) in ice-guarded rear-facing cupolas. They are memory-mapped with the airplane’s anatomic profile. The laser continuously measures (via a mensuration mapping software program) the aircraft’s profile, until it detects an anomaly due to ice accretion. Once armed by this low-power ice-detector laser, this system then commences a thermal lasering of the aircraft’s leading edges, engine intakes, propellers, pitots and forward wing sections. The cupola mounted above the flight deck could handle the empennage and wing areas not visible to the under-nose laser-head..
Such a system might weigh less than the unaerodynamic boots. Lasers are used as airborne mapping tools (WRELADS), range-finding and in other discriminatory tasks (burglar alarms, facial mapping, survey work etc). Thermal lasers are used routinely in welding, eye surgery, offset printing, precision-cutting etc.
Electric power demand for de-icing might not be that great, as heavy-duty capacitors could be charged up over a period of time and then discharged for the periodic phased attacks on ice. As per the standard inflation cycle for de-icer boots, the lasers could phase alternate (top cupola/bottom cupola) and run a 30 seconds on/30 seconds off cycle. Maybe this system could be called the Laissez-faire, a play on the word laser which might appeal to the French manufacturer of the ATR-72 (Laissez-faire = Non-interference in the affairs of others, as in ‘ice go away’).

Miserlou
8th Nov 2003, 02:57
For any-one who doesn't know of this incident, the a/c span from FL180 and impacted the surface 45 seconds later!

It's been a few months since I read the report but I do remember that this incident was caused by incompetance. It took nearly half an hour from the first ice accretion warning until the final manouvre.

The crew appeared not to take any action and couldn't even make a decision to climb or descend. Now, come on. They just sat in the top of a cloud at FL180 watching the airspeed decrease.

Just applying the correct ATR ice procedure would have avoided this incident.

I can't say I understand your theories as to stall AoA's (rather, I understand them but not their relevance) as the stick shaker values both for normal and icing conditions are well below the values you state.

BEagle
8th Nov 2003, 16:40
Miserlou - any aerofoil affected by considerable ice accretion will have a totally different Cl-alpha curve compared with the same aerofoil in its uncontaminated state. The alpha value which will trigger stall protection devices will be set up for the uncontaminated aerofoil, hence it has no 'knowledge' of the stall alpha of the contaminated wing. Thus a wing with considerable icing may well stall at an alpha value below the 'normal' stick-shaker value.

Flying as SLF in an ATR72 twice next week - shall keep an eye out for ice...

Captain Stable
8th Nov 2003, 20:17
I'm not qualified to comment on BEagle's comments, otherwise I tend to agree with Miserlou on the manner in which this aircraft was operated.

An ATR's anti- and de-icing system is very good, when operated in accordance with the FCOM. In particular, when in heavy ice-accreting conditions, the first thing you don't do is sit there waiting while it builds up and up. The second thing you don't do is leave the autopilot in to struggle more and more.

The laser system is, IMO, unneccesary. The ATR's ice detection system works very well, as do the phased boots. I can't comment on the different stall angles under icing conditions, but I've never noticed such a phenomenon in the sim.

Jagbag
8th Nov 2003, 20:45
The ATR is very prone to speed reduction during icing. Problem occurs when you have a multiple emergency along with icing - at which time any additional anti icing equipment available is very welcome. It may be worthwhile to test out the laser deicer specified. The other two I am not so sure about

While carrying out an ETOPS I experienced icing at FL 190 with slight but progressive reduction in speed even after applying full anti and de icing measures. I decided to descend, however due to the large distance was unable to contact area control on R/T for a descent. Luckily managed to relay for lower level clearance - which came thru'.

Moral of the story is to choose cruising level with care, preferably take a lower level. Also don't hesitate to add a little fuel to the departure figure even if it restricts the payload.

Hope this helps.

Miserlou
9th Nov 2003, 22:19
BEagle.

That is the part I understand.

The ATR has two trigger values of AoA for the stick shaker; 12.5 for normal and 7.5 for icing conditions. The Icing AoA comes on when you select level 2 (anti-icing).
The aircraft will also give a Master Caution for Ice Accretion when actual accretion occurs.

You must have seen the ATR ice tests. These show the procedures to be safe.

Belgique
19th Nov 2003, 02:22
The LASER ice-detection and DE-Icing idea sounds as though it would work.

I am more interested in whether the fact that prop rotation direction (being the same on both port and starboard wings) and exhaust positioning could induce a very L&R asymmetrically iced airfoil on both mainplane and tailplane in freezing rain conditions.

It seems to me that whenever these type accidents occur, the fingers of scorn quickly point at the dummies who allowed the situation to develop and/or kept the autopilot plugged in and failed to notice the pitch trim autotrimming itself into an accident.

But what if the icing does build up very asymmetrically and quite fast in rain-ice/freezing rain/SCDD's ? That's supercooled drizzle droplets by any other name. Only someone who's been caught at intermediate levels in a turboprop flying along a warm front at the wrong height would have a real grip on just how fast you can accumulate. It sounds quite logical to me that the icing will take up a different concentration (thickly nacelle inboard on the port side and thickly nacelle-outboard on the starboard side) and that that spanwise distribution of lift will generate a surprisingly strong breakaway roll once the first wing (normally RHS) reaches its much reduced stalling angle.

But no doubt it's something that shall remain a mystery until after the next pax-batch meet their icing-induced fate.

White Knight
21st Nov 2003, 12:25
Have flown ATR 42 and 72 aircraft in moderate and severe icing. Apply the procedures, no problems...:cool: