UNCTUOUS
7th Nov 2003, 23:01
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’).
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’).