Fine Article on Turboprops and Icing
Thread Starter
Join Date: Oct 2007
Location: fort sheridan, il
Posts: 1,656
Likes: 0
Received 0 Likes
on
0 Posts
Fine Article on Turboprops and Icing
as this is NEWS, I think it should be posted and read:
From business and commercial aviation.
Recovering From Ice-induced Stalls in Turboprops
Dec 12, 2007
By Patrick R. Veillette, Ph.D.
When considering inflight icing events, most U.S.-based pilots probably think of the proverbial "icebox" conditions over the Great Lakes, the nasty crud associated with occluded weather fronts and freezing rain conditions over the Central and Northeastern states. Australia or Southern California may seem far removed from such activity, but icing incidents there have actually deteriorated into temporary losses of control.
Surprised? Here are the particulars of one. A Saab 340B departed Sydney for Bathurst, New South Wales. The 35-minute flight was the sixth leg of the day for the flight crew. At the controls were a PIC with 9,530 total flight hours, of which1,939 were in type, and an SIC with 6,620 total flight hours, of which 1,451 hours were in type.
The area forecast indicated the freezing level would be near 4,000 feet and that moderate icing conditions could be expected in clouds above the freezing level. The forecast for Bathurst included snow showers, a surface temperature of 2°C, a broken ceiling of 800 feet and southwesterly winds gusting to 28 knots, necessitating a circling approach that night.
The pilot was flying the aircraft on autopilot and during descent, the twin turboprop entered clouds several times from cruise altitude (12,000 feet) to the initial approach altitude (5,700 feet) then to the minimum descent altitude for the GPS approach (3,810 feet.) The flight crew observed ice accumulating on the windshield wiper but not on the wing leading edges. However, due to conflicting information between the SOPs in the airplane flight manual and the aircraft operating manual, the flight crew activated only the engine anti-ice system and not the surface deicing systems, which included the propeller deice and deice boots on the wing leading edges and on the tail leading edges.
A circling nighttime approach in icing conditions posed a triple threat to the crew, and to make matters worse, the GPS approach was quite steep. So, even with the flaps extended to 20 degrees, landing gear extended and the torque reduced to flight idle, the Saab arrived at the MDA at a speed significantly faster than the normal circling speed of 130 knots. The PIC left the power levers at idle to begin slowing to the appropriate speed while looking outside the aircraft into the darkness and beginning a right-hand turn to track downwind for Runway 17. The SIC pointed out that the airspeed had begun to decay below the target speed, and as the PIC added power and began to roll out of the right turn, the aircraft suddenly rolled left past the vertical and pitched nose-down without warning.
The FDR recorded that the angle of attack (AOA) at the moment of the break was 9.5 degrees, far below the normal critical angle of attack for the wing. Normally the stick shaker wouldn't even activate until 13.1 degrees AOA to warn the pilot of an impending stall. What could have caused this sudden and abrupt stall? The wing leading edges were contaminated with a relatively small but aerodynamically significant one-half inch layer of ice, which caused the left wing to stall so prematurely.
The PIC overpowered the autopilot, aided no doubt by a surge of adrenaline, and rolled the aircraft back from 109 degrees left bank to approximately 35 degrees when the right wing stalled, rolling the aircraft to about 56 degrees right wing down. According to the official accident investigation report (Reference: Australian Transport Safety Bureau, Air Safety Investigation Report 200203074: Inflight Loss of Control Due to Airframe Icing, Saab 340B, VH-OLM, 28 June 2002 ), as the aircraft passed through 688 feet agl, the aircraft's pitch attitude was 19 degrees nose down! The PIC then rolled the aircraft to a wings-level attitude, increased power to 100 percent torque, and applied nose-up pitch inputs.
The good news is that this aircrew luckily recovered the aircraft before it struck the ground. Another crew, this one with American Eagle and flying from San Luis Obispo, Calif., had a similar experience when their Saab 340 encountered ice at 11,000 feet msl during climb-out. The abrupt ice-induced stall resulted in an altitude loss of more than 5,000 feet before the pilots regained control of the aircraft. (See "A Sudden and Long Descent" sidebar.)
Other crews have not been so lucky. In fact, just three ice-induced stalls in turboprop aircraft have led to loss-of-control accidents that killed 134 people. These crashes involved an ATR 42 in Crezzo, Italy, in 1987, an ATR 72 in Roselawn, Ind., in 1994 and an EMB 120 Brasilia in Monroe, Mich., in 1997. In all three accidents the pilots initially did not reduce the wing AOA by moving the control column to the nose-down position early in the upset sequence. This hesitation compounded the severity of the upset, and made recovery more difficult and unlikely.
Was their delayed action aberrant? A NASA-funded simulator study using regional airline pilots as subjects duplicated the events leading up to the loss-of-control in the Roselawn accident and found that fewer than half of the pilots were able to recover the aircraft. According to the report, "The pilots appeared to respond in accordance with their training for excessive bank and stall recovery, but they did not implement corrective actions uniquely required for icing-induced roll and uncommanded control movement. Normal stall-recovery training emphasizes applying maximum power and minimum loss of altitude. In contrast, recovery from icing-induced rolls and more complete stalls requires trading altitude for airspeed."
John P. Dow Sr., the recently retired subject matter expert on icing issues in the FAA's Aircraft Certification Service, emphasizes this significant disparity between the instilled-from-training "normal" stall recovery and a "real world" stall induced by ice on the wing. The vast majority of us were trained in simulators to respond to the first indication of a stall by applying power and maintaining pitch attitude, with the objective of minimizing altitude loss during recovery. In theory, the airplane will accelerate to an increased airspeed and a reduced AOA. By the way, these procedures are somewhat dictated by the FAA's Practical Test Standards, which require recovery to be initiated at the first indication of an impending stall, as well as minimizing altitude loss.
However, a "real world" ice-induced stall is preceded by little warning, if any. During its investigation of the incident at Bathurst, Australian authorities identified five other serious incidents involving Saab 340s in which trace to light amounts of icing led to premature stalls with little or no warning to flight crews.
The first effect of ice accumulation usually is a reduction of the stall AOA, which in turn causes a corresponding increase in the stall speed. If a flight crew unwittingly allows the aircraft to reach this ice-induced critical AOA, what's likely to follow will be an abrupt uncommanded roll, buffet or other aerodynamic cues without activating the stick shaker. The ice-induced reduction in the wing's critical AOA, that is the point at which the wing is considered to stall, and the simultaneous increase in the stall speed can be significant. Data collected from a British Aerospace ATP that was involved in an icing upset in 1991 determined that the ice-induced stall occurred at about 140 knots, compared with a normal stall speed of about 110 knots. For you aerodynamicists, the wing stalled at a lift coefficient of 0.9, rather than the nearly 1.6 coefficient for an uncontaminated wing.
There was a perception in the aftermath of the Roselawn accident that such abrupt roll upsets occur mainly in severe icing conditions. However, insidious and difficult-to-see icing accumulations appear to have caused many of the roll upsets. According to George Bershinsky, pilot of the University of Wyoming's King Air being used in icing research for the National Center for Atmospheric Research (NCAR), "less than one-sixteenth inch of icing can reduce a wing's lift by 25 percent. This little is sometimes hard to see, but the stall speed increases by around 20 percent."
It is important to realize that the wing's critical AOA may change with no apparent visual, tactile or performance cues associated with the icing condition. In many of the ice-induced accidents as well as the inflight icing research conducted by agencies such as NASA-Glenn Research Center and the NCAR, the airplane provided no advanced warning to the pilot. NCAR test pilots noted airplane response and kinesthetic cues to an ice-related stall can be substantially different from the simulator training scenario. Given this information, it shouldn't be surprising that there have been accidents in which the stall and upset occurred prior to stick shaker activation in ice-contaminated airplanes.
According to Jon Hannan, former flight test pilot for the FAA's Small Airplane Directorate, "The pilot should hand fly the airplane in icing conditions that are severe enough to effect a slowdown because mushy control feel and/or small oscillations usually can be felt in time to recover prior to a stall." By contrast, with the autopilot engaged, the pilot cannot feel the responsiveness of the flight controls. Moreover, as ice accumulates, the autopilot might be required to trim the controls against the adverse effects of the ice. An instantaneous and substantial control input might surprise the pilot if the autopilot reaches its trim-force limits and disengages unexpectedly. The pilot might be similarly surprised by the aircraft's reaction upon intentionally disengaging the autopilot and discovering it had been masking control inputs.
As to why stall warning systems fail to provide alerts in icing conditions, Dow explains, "There are an infinite variety of shapes, thicknesses and textures of ice that can accrete at various locations on the airfoil. Each ice shape essentially produces a new airfoil with unique lift, drag, stall angle and pitching moment characteristics that are different from the host airfoil, and from other ice shapes. There is a range of effects from these shapes. Some effects are relatively benign and are almost indistinguishable from the host airfoil. Others may alter the aerodynamic characteristics so drastically that all or part of the airfoil stalls suddenly and without warning. Sometimes the difference in ice accretion between a benign shape and a more hazardous shape appears relatively insignificant. There is no way for the pilot to know what the resulting stall AOA is at any given time."
Some stall warning systems are calibrated with reference to a dry, uncontaminated wing, while some others, such as Safe Flight's Angle of Attack/Stall Warning System, incorporates dual modes, one for normal conditions and one for icing conditions. In normal mode, the stick shaker activation, AOA meter, AOA indexer and low airspeed awareness are all referenced to standard airplane stall speeds (think "dry uncontaminated airfoil"). In the Ice Mode, these are all referenced to the standard airplane stall speeds plus five knots. This is to account for residual airframe ice present during or after an icing encounter. In the air, the Ice Mode is activated in some aircraft when either or both engine anti-ice switches are ON.
A second and very significant pitfall in the present stall practice and recovery methodology is that attempting to complete the recovery with minimum altitude loss may actually compound the altitude loss and further inhibit control. According to Daniel Meier Jr., aviation safety inspector, flight operations, FAA headquarters, "A stall caused by icing is extremely hazardous because you cannot conserve altitude by maintaining attitude. Adhering to the standard of minimum altitude loss ingrained in training has resulted in pilots failing to recover from ice-related stalls and upsets that have resulted in altitude losses in excess of 5,000 feet in turboprop airplanes. Pilots of turboprop airplanes should be taught that they might need to trade some altitude for airspeed if the airplane stalls during flight in icing conditions."
To make matters worse, recovery from an icing-induced stall in a turboprop is adversely affected by the effect of ice on the propellers. Prop blades begin to stall at the large blade-pitch angles associated with high torque values, and thus produce substantially less than 100 percent thrust -- in some events, as low as 85 percent thrust. Moreover, ice on unprotected surfaces of the propeller blades also reduces thrust. Tests have shown that this un-clearable contamination can reduce thrust by 20 percent. Even without partial propeller blade stall, reduced thrust by ice on the propeller blades combined with increased ice-related drag on the rest of the airframe is an overall double penalty for a turboprop airplane.
Most of these ice-induced stalls have resulted in severe roll upsets, which is due in part to the effects of propwash and the tendency of the stall to start at the wingtip. The airfoil at the wingtip is often a different shape than that at the root. It is probably thinner, has a different camber, shorter chord and two or three degrees of twist or washout relative to the root section. Because the tip section may have a sharper nose radius and shorter chord, it is a more efficient ice collector. As a result of the differences cited, ice accretion at the wingtip may be thicker, extend farther aft and have a greater adverse effect than ice at the root. Greater ice accretion will probably occur at the wingtip, leaving it more impaired aerodynamically than the inboard wing sections. Thus an ice-induced stall, instead of starting inboard, may start at the tip.
Power (propwash) effects can aggravate tip-stall since it reduces the AOA of the section of the wing directly behind the disc. At high power settings, stall on the inner wing tends to be delayed by propwash, but the outer wing doesn't see the same flow field, so it tends to stall sooner. Unless the propellers are counter-rotating, the flow field is asymmetric over the wings and ice impingement tends to be slightly asymmetric as well.
Because the propwash strongly influences the airflow over the wing immediately behind the prop, it should be no surprise that changing power has induced some of the ice-related upsets, several of which occurred at the top of decent, most likely when power was reduced.
While the foregoing focuses on the vulnerabilities of turboprops, jets are not immune. However, jets tend to spend less time exposed to icing conditions since they generally operate at altitudes above most icing conditions for a greater percentage of their flight time.
And turboprops are hindered in the icing environment by yet another factor. In a paper entitled "Aircraft Icing Problems -- After 50 Years," Porter Perkins and William Rieke, researchers at NASA-Glenn, state, "Light icing reported by a jet aircraft with ice protection can be moderate to severe when the same icing is encountered by a turboprop aircraft." So, not only does the average turboprop tend to spend more time in the icing environment, it also tends to pick up ice much quicker than jets.
According to Dow, an immediate recovery from an ice-related stall is most likely when:
(1) At the first sign of a stall, the pilot applies nose-down pitch control and levels the wings while increasing prop speed and torque until a sufficient increase in airspeed (and decrease in AOA) is attained. In most events, the nose will drop as a consequence of the stall, but it will result in an insufficient decrease in AOA, requiring further nose-down pitch change.
(2) If the nose cannot be lowered, extend the flaps from the cruise configuration to the first setting and then lower the nose to increase speed as appropriate. After recovering, retract the flaps when appropriate.
The importance of lowering the nose to reduce the AOA is vital in the recovery as a 1998 incident involving a Brasilia illustrates well. During the onset of the upset the aircraft abruptly rolled about 65 degrees right and then about 45 degrees left. The pilot was unable to recover by increasing power to 100 percent and maintaining pitch attitude. In fact, the initial power increase to 100 percent torque and a momentary increase to nearly 150 percent torque did not increase airspeed sufficiently to enable recovery. It was only after extending the flaps to the approach setting, which increased the lift coefficient, and lowering the nose, which reduced the AOA, that the pilot was able to reestablish control.
According to the FAA's Airplane Upset Recovery Training Aid, upset recovery education must not include simulator testing criteria since that "could lead to similar negative learning conclusions that can currently exist with approach to stall performance when measured against minimum loss of altitude." It also offers this piece of irrefutable wisdom: "Avoidance of environmentally induced upsets is the best course of action. Pilots should monitor the environmental conditions and avoid high-risk situations."
Incidentally, the "Safer Skies" initiative formed in 1997 to reduce aviation fatalities in both general and commercial aviation determined that among the ways to reduce loss-of-control accidents is stricter criteria for flight-into-known-icing conditions.
While you might not think the typically sunny skies of Southern California and Australia would produce inflight icing lessons, they have. And we need to learn from them and take steps to avoid their repetition. Because the next outcome could be far different. B&CA
From business and commercial aviation.
Recovering From Ice-induced Stalls in Turboprops
Dec 12, 2007
By Patrick R. Veillette, Ph.D.
When considering inflight icing events, most U.S.-based pilots probably think of the proverbial "icebox" conditions over the Great Lakes, the nasty crud associated with occluded weather fronts and freezing rain conditions over the Central and Northeastern states. Australia or Southern California may seem far removed from such activity, but icing incidents there have actually deteriorated into temporary losses of control.
Surprised? Here are the particulars of one. A Saab 340B departed Sydney for Bathurst, New South Wales. The 35-minute flight was the sixth leg of the day for the flight crew. At the controls were a PIC with 9,530 total flight hours, of which1,939 were in type, and an SIC with 6,620 total flight hours, of which 1,451 hours were in type.
The area forecast indicated the freezing level would be near 4,000 feet and that moderate icing conditions could be expected in clouds above the freezing level. The forecast for Bathurst included snow showers, a surface temperature of 2°C, a broken ceiling of 800 feet and southwesterly winds gusting to 28 knots, necessitating a circling approach that night.
The pilot was flying the aircraft on autopilot and during descent, the twin turboprop entered clouds several times from cruise altitude (12,000 feet) to the initial approach altitude (5,700 feet) then to the minimum descent altitude for the GPS approach (3,810 feet.) The flight crew observed ice accumulating on the windshield wiper but not on the wing leading edges. However, due to conflicting information between the SOPs in the airplane flight manual and the aircraft operating manual, the flight crew activated only the engine anti-ice system and not the surface deicing systems, which included the propeller deice and deice boots on the wing leading edges and on the tail leading edges.
A circling nighttime approach in icing conditions posed a triple threat to the crew, and to make matters worse, the GPS approach was quite steep. So, even with the flaps extended to 20 degrees, landing gear extended and the torque reduced to flight idle, the Saab arrived at the MDA at a speed significantly faster than the normal circling speed of 130 knots. The PIC left the power levers at idle to begin slowing to the appropriate speed while looking outside the aircraft into the darkness and beginning a right-hand turn to track downwind for Runway 17. The SIC pointed out that the airspeed had begun to decay below the target speed, and as the PIC added power and began to roll out of the right turn, the aircraft suddenly rolled left past the vertical and pitched nose-down without warning.
The FDR recorded that the angle of attack (AOA) at the moment of the break was 9.5 degrees, far below the normal critical angle of attack for the wing. Normally the stick shaker wouldn't even activate until 13.1 degrees AOA to warn the pilot of an impending stall. What could have caused this sudden and abrupt stall? The wing leading edges were contaminated with a relatively small but aerodynamically significant one-half inch layer of ice, which caused the left wing to stall so prematurely.
The PIC overpowered the autopilot, aided no doubt by a surge of adrenaline, and rolled the aircraft back from 109 degrees left bank to approximately 35 degrees when the right wing stalled, rolling the aircraft to about 56 degrees right wing down. According to the official accident investigation report (Reference: Australian Transport Safety Bureau, Air Safety Investigation Report 200203074: Inflight Loss of Control Due to Airframe Icing, Saab 340B, VH-OLM, 28 June 2002 ), as the aircraft passed through 688 feet agl, the aircraft's pitch attitude was 19 degrees nose down! The PIC then rolled the aircraft to a wings-level attitude, increased power to 100 percent torque, and applied nose-up pitch inputs.
The good news is that this aircrew luckily recovered the aircraft before it struck the ground. Another crew, this one with American Eagle and flying from San Luis Obispo, Calif., had a similar experience when their Saab 340 encountered ice at 11,000 feet msl during climb-out. The abrupt ice-induced stall resulted in an altitude loss of more than 5,000 feet before the pilots regained control of the aircraft. (See "A Sudden and Long Descent" sidebar.)
Other crews have not been so lucky. In fact, just three ice-induced stalls in turboprop aircraft have led to loss-of-control accidents that killed 134 people. These crashes involved an ATR 42 in Crezzo, Italy, in 1987, an ATR 72 in Roselawn, Ind., in 1994 and an EMB 120 Brasilia in Monroe, Mich., in 1997. In all three accidents the pilots initially did not reduce the wing AOA by moving the control column to the nose-down position early in the upset sequence. This hesitation compounded the severity of the upset, and made recovery more difficult and unlikely.
Was their delayed action aberrant? A NASA-funded simulator study using regional airline pilots as subjects duplicated the events leading up to the loss-of-control in the Roselawn accident and found that fewer than half of the pilots were able to recover the aircraft. According to the report, "The pilots appeared to respond in accordance with their training for excessive bank and stall recovery, but they did not implement corrective actions uniquely required for icing-induced roll and uncommanded control movement. Normal stall-recovery training emphasizes applying maximum power and minimum loss of altitude. In contrast, recovery from icing-induced rolls and more complete stalls requires trading altitude for airspeed."
John P. Dow Sr., the recently retired subject matter expert on icing issues in the FAA's Aircraft Certification Service, emphasizes this significant disparity between the instilled-from-training "normal" stall recovery and a "real world" stall induced by ice on the wing. The vast majority of us were trained in simulators to respond to the first indication of a stall by applying power and maintaining pitch attitude, with the objective of minimizing altitude loss during recovery. In theory, the airplane will accelerate to an increased airspeed and a reduced AOA. By the way, these procedures are somewhat dictated by the FAA's Practical Test Standards, which require recovery to be initiated at the first indication of an impending stall, as well as minimizing altitude loss.
However, a "real world" ice-induced stall is preceded by little warning, if any. During its investigation of the incident at Bathurst, Australian authorities identified five other serious incidents involving Saab 340s in which trace to light amounts of icing led to premature stalls with little or no warning to flight crews.
The first effect of ice accumulation usually is a reduction of the stall AOA, which in turn causes a corresponding increase in the stall speed. If a flight crew unwittingly allows the aircraft to reach this ice-induced critical AOA, what's likely to follow will be an abrupt uncommanded roll, buffet or other aerodynamic cues without activating the stick shaker. The ice-induced reduction in the wing's critical AOA, that is the point at which the wing is considered to stall, and the simultaneous increase in the stall speed can be significant. Data collected from a British Aerospace ATP that was involved in an icing upset in 1991 determined that the ice-induced stall occurred at about 140 knots, compared with a normal stall speed of about 110 knots. For you aerodynamicists, the wing stalled at a lift coefficient of 0.9, rather than the nearly 1.6 coefficient for an uncontaminated wing.
There was a perception in the aftermath of the Roselawn accident that such abrupt roll upsets occur mainly in severe icing conditions. However, insidious and difficult-to-see icing accumulations appear to have caused many of the roll upsets. According to George Bershinsky, pilot of the University of Wyoming's King Air being used in icing research for the National Center for Atmospheric Research (NCAR), "less than one-sixteenth inch of icing can reduce a wing's lift by 25 percent. This little is sometimes hard to see, but the stall speed increases by around 20 percent."
It is important to realize that the wing's critical AOA may change with no apparent visual, tactile or performance cues associated with the icing condition. In many of the ice-induced accidents as well as the inflight icing research conducted by agencies such as NASA-Glenn Research Center and the NCAR, the airplane provided no advanced warning to the pilot. NCAR test pilots noted airplane response and kinesthetic cues to an ice-related stall can be substantially different from the simulator training scenario. Given this information, it shouldn't be surprising that there have been accidents in which the stall and upset occurred prior to stick shaker activation in ice-contaminated airplanes.
According to Jon Hannan, former flight test pilot for the FAA's Small Airplane Directorate, "The pilot should hand fly the airplane in icing conditions that are severe enough to effect a slowdown because mushy control feel and/or small oscillations usually can be felt in time to recover prior to a stall." By contrast, with the autopilot engaged, the pilot cannot feel the responsiveness of the flight controls. Moreover, as ice accumulates, the autopilot might be required to trim the controls against the adverse effects of the ice. An instantaneous and substantial control input might surprise the pilot if the autopilot reaches its trim-force limits and disengages unexpectedly. The pilot might be similarly surprised by the aircraft's reaction upon intentionally disengaging the autopilot and discovering it had been masking control inputs.
As to why stall warning systems fail to provide alerts in icing conditions, Dow explains, "There are an infinite variety of shapes, thicknesses and textures of ice that can accrete at various locations on the airfoil. Each ice shape essentially produces a new airfoil with unique lift, drag, stall angle and pitching moment characteristics that are different from the host airfoil, and from other ice shapes. There is a range of effects from these shapes. Some effects are relatively benign and are almost indistinguishable from the host airfoil. Others may alter the aerodynamic characteristics so drastically that all or part of the airfoil stalls suddenly and without warning. Sometimes the difference in ice accretion between a benign shape and a more hazardous shape appears relatively insignificant. There is no way for the pilot to know what the resulting stall AOA is at any given time."
Some stall warning systems are calibrated with reference to a dry, uncontaminated wing, while some others, such as Safe Flight's Angle of Attack/Stall Warning System, incorporates dual modes, one for normal conditions and one for icing conditions. In normal mode, the stick shaker activation, AOA meter, AOA indexer and low airspeed awareness are all referenced to standard airplane stall speeds (think "dry uncontaminated airfoil"). In the Ice Mode, these are all referenced to the standard airplane stall speeds plus five knots. This is to account for residual airframe ice present during or after an icing encounter. In the air, the Ice Mode is activated in some aircraft when either or both engine anti-ice switches are ON.
A second and very significant pitfall in the present stall practice and recovery methodology is that attempting to complete the recovery with minimum altitude loss may actually compound the altitude loss and further inhibit control. According to Daniel Meier Jr., aviation safety inspector, flight operations, FAA headquarters, "A stall caused by icing is extremely hazardous because you cannot conserve altitude by maintaining attitude. Adhering to the standard of minimum altitude loss ingrained in training has resulted in pilots failing to recover from ice-related stalls and upsets that have resulted in altitude losses in excess of 5,000 feet in turboprop airplanes. Pilots of turboprop airplanes should be taught that they might need to trade some altitude for airspeed if the airplane stalls during flight in icing conditions."
To make matters worse, recovery from an icing-induced stall in a turboprop is adversely affected by the effect of ice on the propellers. Prop blades begin to stall at the large blade-pitch angles associated with high torque values, and thus produce substantially less than 100 percent thrust -- in some events, as low as 85 percent thrust. Moreover, ice on unprotected surfaces of the propeller blades also reduces thrust. Tests have shown that this un-clearable contamination can reduce thrust by 20 percent. Even without partial propeller blade stall, reduced thrust by ice on the propeller blades combined with increased ice-related drag on the rest of the airframe is an overall double penalty for a turboprop airplane.
Most of these ice-induced stalls have resulted in severe roll upsets, which is due in part to the effects of propwash and the tendency of the stall to start at the wingtip. The airfoil at the wingtip is often a different shape than that at the root. It is probably thinner, has a different camber, shorter chord and two or three degrees of twist or washout relative to the root section. Because the tip section may have a sharper nose radius and shorter chord, it is a more efficient ice collector. As a result of the differences cited, ice accretion at the wingtip may be thicker, extend farther aft and have a greater adverse effect than ice at the root. Greater ice accretion will probably occur at the wingtip, leaving it more impaired aerodynamically than the inboard wing sections. Thus an ice-induced stall, instead of starting inboard, may start at the tip.
Power (propwash) effects can aggravate tip-stall since it reduces the AOA of the section of the wing directly behind the disc. At high power settings, stall on the inner wing tends to be delayed by propwash, but the outer wing doesn't see the same flow field, so it tends to stall sooner. Unless the propellers are counter-rotating, the flow field is asymmetric over the wings and ice impingement tends to be slightly asymmetric as well.
Because the propwash strongly influences the airflow over the wing immediately behind the prop, it should be no surprise that changing power has induced some of the ice-related upsets, several of which occurred at the top of decent, most likely when power was reduced.
While the foregoing focuses on the vulnerabilities of turboprops, jets are not immune. However, jets tend to spend less time exposed to icing conditions since they generally operate at altitudes above most icing conditions for a greater percentage of their flight time.
And turboprops are hindered in the icing environment by yet another factor. In a paper entitled "Aircraft Icing Problems -- After 50 Years," Porter Perkins and William Rieke, researchers at NASA-Glenn, state, "Light icing reported by a jet aircraft with ice protection can be moderate to severe when the same icing is encountered by a turboprop aircraft." So, not only does the average turboprop tend to spend more time in the icing environment, it also tends to pick up ice much quicker than jets.
According to Dow, an immediate recovery from an ice-related stall is most likely when:
(1) At the first sign of a stall, the pilot applies nose-down pitch control and levels the wings while increasing prop speed and torque until a sufficient increase in airspeed (and decrease in AOA) is attained. In most events, the nose will drop as a consequence of the stall, but it will result in an insufficient decrease in AOA, requiring further nose-down pitch change.
(2) If the nose cannot be lowered, extend the flaps from the cruise configuration to the first setting and then lower the nose to increase speed as appropriate. After recovering, retract the flaps when appropriate.
The importance of lowering the nose to reduce the AOA is vital in the recovery as a 1998 incident involving a Brasilia illustrates well. During the onset of the upset the aircraft abruptly rolled about 65 degrees right and then about 45 degrees left. The pilot was unable to recover by increasing power to 100 percent and maintaining pitch attitude. In fact, the initial power increase to 100 percent torque and a momentary increase to nearly 150 percent torque did not increase airspeed sufficiently to enable recovery. It was only after extending the flaps to the approach setting, which increased the lift coefficient, and lowering the nose, which reduced the AOA, that the pilot was able to reestablish control.
According to the FAA's Airplane Upset Recovery Training Aid, upset recovery education must not include simulator testing criteria since that "could lead to similar negative learning conclusions that can currently exist with approach to stall performance when measured against minimum loss of altitude." It also offers this piece of irrefutable wisdom: "Avoidance of environmentally induced upsets is the best course of action. Pilots should monitor the environmental conditions and avoid high-risk situations."
Incidentally, the "Safer Skies" initiative formed in 1997 to reduce aviation fatalities in both general and commercial aviation determined that among the ways to reduce loss-of-control accidents is stricter criteria for flight-into-known-icing conditions.
While you might not think the typically sunny skies of Southern California and Australia would produce inflight icing lessons, they have. And we need to learn from them and take steps to avoid their repetition. Because the next outcome could be far different. B&CA
Join Date: Feb 2001
Location: UK
Posts: 162
Likes: 0
Received 0 Likes
on
0 Posts
NASA Research
A good piece and quite right to post it.
The whole inflight icing issue is a gigantic one.
Even in the large regional airlines I have operated in I have found distinct deficiencies in the calibre of understanding of inflight icing and icing induced stall recovery....even within the training department.
To be fair to the industry at large we as professional pilots have a resposibility to demand access from our employers to the resources we all need to expand our knowledge and training in this area as in many others rather than waiting for the authorities to catch up and implement/modify minimum training standards. It would help if our unions took a more active role in technical vitality/professional development as well. Sadly my union magazine looks more like a guide to retirement and holiday planning...nice glossy photos though. Maybe a cheaper production with more time and money spent on TECHNICAL content like the medics and lawyers have would be good. (BUMP........."fell of the damn soapbox!")
But I digress.....
NASA continue to provide some fantastic resources to regional (jet and turboprop) pilots to assist in the diagnosis and correct response to icing induced events/stalls. I have attached a good link to some free tutorials on the subject. This advice is all evidence and experiment derived so the basis is good for taking it onboard. Sadly anecdotes prevail in this area and are too commonly given more credence than they deserve!
If pilots take on board anything from this it's important to recognise the important differences between tailplane icing induced upset diagnosis/recovery and wing icing induced stall upset diagnosis/recovery. There are differences but they should become clear after the tutorial.
Happy reading.
http://icebox.grc.nasa.gov/education/index.html
The whole inflight icing issue is a gigantic one.
Even in the large regional airlines I have operated in I have found distinct deficiencies in the calibre of understanding of inflight icing and icing induced stall recovery....even within the training department.
To be fair to the industry at large we as professional pilots have a resposibility to demand access from our employers to the resources we all need to expand our knowledge and training in this area as in many others rather than waiting for the authorities to catch up and implement/modify minimum training standards. It would help if our unions took a more active role in technical vitality/professional development as well. Sadly my union magazine looks more like a guide to retirement and holiday planning...nice glossy photos though. Maybe a cheaper production with more time and money spent on TECHNICAL content like the medics and lawyers have would be good. (BUMP........."fell of the damn soapbox!")
But I digress.....
NASA continue to provide some fantastic resources to regional (jet and turboprop) pilots to assist in the diagnosis and correct response to icing induced events/stalls. I have attached a good link to some free tutorials on the subject. This advice is all evidence and experiment derived so the basis is good for taking it onboard. Sadly anecdotes prevail in this area and are too commonly given more credence than they deserve!
If pilots take on board anything from this it's important to recognise the important differences between tailplane icing induced upset diagnosis/recovery and wing icing induced stall upset diagnosis/recovery. There are differences but they should become clear after the tutorial.
Happy reading.
http://icebox.grc.nasa.gov/education/index.html
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
Severe icing with turbopropellor aircraft and the proper recovery procedures to be used was well known long ago, after the Viscount tailplane icing accident many years ago.
However, when the experienced pilots of the time retired, this knowledge was lost, and of course the problems surfaced once again many years later, with the Roselawn ATR accident.
About the only turbopropeller aircraft that was not affected by severe icing was an oldie...the Lockheed 188 Electra.
Fondly remembered, never equaled.
And why not...it was a Lockheed.
However, when the experienced pilots of the time retired, this knowledge was lost, and of course the problems surfaced once again many years later, with the Roselawn ATR accident.
About the only turbopropeller aircraft that was not affected by severe icing was an oldie...the Lockheed 188 Electra.
Fondly remembered, never equaled.
And why not...it was a Lockheed.
Join Date: Mar 2000
Location: Obvious
Age: 78
Posts: 301
Likes: 0
Received 0 Likes
on
0 Posts
A Few Missed Points I feel....
No real recognition here of the two turboprops rotating in the same direction and its inevitable effect upon asymmetry in the spanwise build of icing deposits (and consequent asymmetry of wings leading to different stalling angles of attack on port and starboard wings ==> rapid-rate sudden roll).
.
Additionally, no emphasis upon the instinctive pilot reaction of opposite aileron (vice the non-instinctive nose-lowering/reducing AoA) - or the totally exacerbating effect of "opposite aileron" upon the "unrecognized" auto-rotative roll-rate. In this respect it's similar to a V-22 Osprey pilot using "opposite aileron" (i.e. differential collective) once embedded in asymmetric Vortex Ring (AsymmVR)..... to simply "pick up" a dropped wing.
.
Additionally, no emphasis upon the instinctive pilot reaction of opposite aileron (vice the non-instinctive nose-lowering/reducing AoA) - or the totally exacerbating effect of "opposite aileron" upon the "unrecognized" auto-rotative roll-rate. In this respect it's similar to a V-22 Osprey pilot using "opposite aileron" (i.e. differential collective) once embedded in asymmetric Vortex Ring (AsymmVR)..... to simply "pick up" a dropped wing.
Join Date: Oct 2000
Location: UK
Posts: 125
Likes: 0
Received 0 Likes
on
0 Posts
411A mentions the Viscount stalling problems.
In fact, the real problem was tailplane stalling when flaps were LOWERED. This action needed more negative lift from the tail which itself stalled as more rearward pressure was applied to the control column. No anti-ice, or minimal temperature resulted in a severe and immediate nose drop. In the Swedish indident, as flap was lowered at the OM, the resultant nose drop was so severe that the hole in the ground was half-way between the OM and threshhold. The tail fell off forwards! Both pilots were ex military fast jet types who sadly seemed to have overlooked the trap into which they flew.
The only real antidote was to use the ant-ice system as described in the book, i.e., ensure that the tailplane temperature was ALWAYS above 50 C.
The big difference between the Viscount and most current turboprops was that the ant-ice system was that heat exchangers supplied hot air into the wing and tailplane leading edges. An excellent system when used correctly but an accident looking to happen if not used properly.
In fact, the real problem was tailplane stalling when flaps were LOWERED. This action needed more negative lift from the tail which itself stalled as more rearward pressure was applied to the control column. No anti-ice, or minimal temperature resulted in a severe and immediate nose drop. In the Swedish indident, as flap was lowered at the OM, the resultant nose drop was so severe that the hole in the ground was half-way between the OM and threshhold. The tail fell off forwards! Both pilots were ex military fast jet types who sadly seemed to have overlooked the trap into which they flew.
The only real antidote was to use the ant-ice system as described in the book, i.e., ensure that the tailplane temperature was ALWAYS above 50 C.
The big difference between the Viscount and most current turboprops was that the ant-ice system was that heat exchangers supplied hot air into the wing and tailplane leading edges. An excellent system when used correctly but an accident looking to happen if not used properly.
Aviator
Join Date: May 2001
Location: Norveg
Posts: 483
Likes: 0
Received 0 Likes
on
0 Posts
The mentioned article "Aircraft Icing Problems -- After 50 Years," is not available electronically from NASA. Anyone who has a copy that would care to share? Forum or PM.
Join Date: Jan 2004
Location: The Moon
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
I have read the article on the AE flight before, and one thing springs to mind if you read the whole thing.
At the time, AE SOP's did not call for pilots sing autopilot or FD to use IAS climb. They, as I recall, were using VS as their climb target. I don;t need to go into the problems with this method I'm sure.
I am working in the USA on the saab and it is in our manual that the only approved method for climb procedure it to use IAS in a climb, when in IMC. You can see why it makes sense, because if you try to hold a VS in icing conditions, the AOA will keep incrasing. The article in question, by quoting two seperate saab incidents, makes the aircraft look like it has a fault. Just my humble opinion, but I think once the lesson was learnt, airlines adopting the IAS climb as their SOP will be fine.
Of course tha doesn't explain the first incident, but using just engine anti-ice might have been the cause there. When the windscreen is iced up, we would be at level 3 anti ice, which has engines, wings and screens on.
I have no basis for comparison with other A/C, but the saab a wonderful aircraft to fly. I really don't see how TP's can be made out to be unsafe as a result of these few reports.
At the time, AE SOP's did not call for pilots sing autopilot or FD to use IAS climb. They, as I recall, were using VS as their climb target. I don;t need to go into the problems with this method I'm sure.
I am working in the USA on the saab and it is in our manual that the only approved method for climb procedure it to use IAS in a climb, when in IMC. You can see why it makes sense, because if you try to hold a VS in icing conditions, the AOA will keep incrasing. The article in question, by quoting two seperate saab incidents, makes the aircraft look like it has a fault. Just my humble opinion, but I think once the lesson was learnt, airlines adopting the IAS climb as their SOP will be fine.
Of course tha doesn't explain the first incident, but using just engine anti-ice might have been the cause there. When the windscreen is iced up, we would be at level 3 anti ice, which has engines, wings and screens on.
I have no basis for comparison with other A/C, but the saab a wonderful aircraft to fly. I really don't see how TP's can be made out to be unsafe as a result of these few reports.
Boris: Yes. In many jets in icing conditions, when the landing gear are extended we also push the tail de-ice button. Then we go Flaps 25 and 40.
SevenStrokeRoll:
Did your quite interesting information or your quotes from the FAA people mention the fact that after two ATR-42s lost control in Europe in icing conditions, the FAA knew about the dangers and kept it quite classified? Secret?
Inflight de-icing testing was done with an Air Force KC-135 which sprayed water as an ATR flew behind it.
Flaws in the FAA de-icing certification requirements became quite apparent.
This was described by either "Aviation Week & ST" or "Flying" magazine after the pulverized bodies were removed from the cornfield in Roselawn, IN.
The FO was the son of one of our old Captains.
Way to go, FAA
! Nice 'alleged' 
cover-up.
The (US) FAA Administrators-our partners promoting aviation. Or should it not be safety? Oh well, lawyers are almost always in charge. The best "ticket-punching" (careerist) liars which money or White House Presidential appointments can buy.
SevenStrokeRoll:
Did your quite interesting information or your quotes from the FAA people mention the fact that after two ATR-42s lost control in Europe in icing conditions, the FAA knew about the dangers and kept it quite classified? Secret?
Inflight de-icing testing was done with an Air Force KC-135 which sprayed water as an ATR flew behind it.
Flaws in the FAA de-icing certification requirements became quite apparent.
This was described by either "Aviation Week & ST" or "Flying" magazine after the pulverized bodies were removed from the cornfield in Roselawn, IN.
The FO was the son of one of our old Captains.
Way to go, FAA
! Nice 'alleged' cover-up.The (US) FAA Administrators-our partners promoting aviation. Or should it not be safety? Oh well, lawyers are almost always in charge. The best "ticket-punching" (careerist) liars which money or White House Presidential appointments can buy.
de minimus non curat lex
I think the morale of the story is quite simple for Saabs. Turn the boots on at the first sign of icing which usually is indicated by ice forming of the wiper arm; or, small ice cystals forming on the lower part of the windscreen.
Works well in Scotland !
Works well in Scotland !
Last edited by parkfell; 15th Dec 2007 at 15:11.
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
Oddly enough, Ignition Override, many larger jet transports do not have any tail anti-icing, the L1011 among them.
Simply not needed.
Now, looking back at the original ATR design, I think you will find that specific relevant icing info was withheld from the FAA, yet the original design, when certified by the FAA, passed all normal icing trials then required.
Look to the ATR manufacturer for the inflight icing problems, not the FAA.
Simply not needed.
Now, looking back at the original ATR design, I think you will find that specific relevant icing info was withheld from the FAA, yet the original design, when certified by the FAA, passed all normal icing trials then required.
Look to the ATR manufacturer for the inflight icing problems, not the FAA.
Join Date: Aug 2002
Location: England.
Posts: 440
Likes: 0
Received 0 Likes
on
0 Posts
About the only turbopropeller aircraft that was not affected by severe icing was an oldie...the Lockheed 188 Electra.
Fondly remembered, never equaled.
And why not...it was a Lockheed.
Fondly remembered, never equaled.
And why not...it was a Lockheed.
-
Prior to modification, the engine mounts permitted excessive precessional movement of the propellers at a critical frequency which occurred at the wings' natural resonance frequency, eventually leading to the separation of the wing from the fuselage.
-
Two fatal crashes in 59 and 60.
-
Ice doesn't accumulate as easily on a violently fluttering wing......and any accumulation on a wing which has fallen off is a tad academic!
-
-
-
Closer to the original topic.....
-
Correct me if I'm wrong, but as far as I know anti/de ice systems consist of electrically heated pads, air bleed heated skin or inflating boots. Use them as the applicable Manual instructs. If two Manuals disagree, then sort out why and get them corrected (it seems that this glaringly obvious action didn't happen in the leading article of this Thread).
-
Anti icing and de icing are different things.
-
Use of boots before a specified ice thickness has accumulated can lead to "inflation" of the ice layer (as if it were a balloon) creating an air space between the boot and ice after boot deflation, rendering further boot operation ineffective. But, obviously, leave it too late and the aircraft stops flying. Requires careful observation and judgement (but then that's piloting for you).
-
Some previous comments by others leave me wondering whether I'm mistaken on some or all of the above, or they are. I'm always willing to learn.
Join Date: Oct 2000
Location: UK
Posts: 125
Likes: 0
Received 0 Likes
on
0 Posts
acbus1
You are absolutely right. I didn't want to go into de-ice and ant-ice differences because of potential thread creep however, I was sorely tempted when parkfell waded in with a somewhat cavalier "turn the boots on at the first sign of icing". Perhaps the experience gap is at work again. It is imperative that the FM usage is adhered. Ice kills.
PS Don't forget the TKS system which works well when needed.
You are absolutely right. I didn't want to go into de-ice and ant-ice differences because of potential thread creep however, I was sorely tempted when parkfell waded in with a somewhat cavalier "turn the boots on at the first sign of icing". Perhaps the experience gap is at work again. It is imperative that the FM usage is adhered. Ice kills.
PS Don't forget the TKS system which works well when needed.
de minimus non curat lex
Boris may be interested to know that the Saab SOP is to operate the boots at the first sign of icing.
Although waiting for say 1/4" to build up on the leading edges first may be acceptable for some types, it is not approved on Saab.
I know TKS systems well; I was involved in some of the TKS trials prior to CAA approval.
Although waiting for say 1/4" to build up on the leading edges first may be acceptable for some types, it is not approved on Saab.
I know TKS systems well; I was involved in some of the TKS trials prior to CAA approval.
Last edited by parkfell; 15th Dec 2007 at 15:13.
Join Date: Feb 2005
Location: Sweden
Age: 77
Posts: 37
Likes: 0
Received 0 Likes
on
0 Posts
Tail Ice
<Quote>
411A mentions the Viscount stalling problems.
In fact, the real problem was tailplane stalling when flaps were LOWERED. This action needed more negative lift from the tail which itself stalled as more rearward pressure was applied to the control column. No anti-ice, or minimal temperature resulted in a severe and immediate nose drop.</Quote>
A similar thing actually happened to the SAAB 340 during I believe the second or third year of operation. During an approach in the north of Sweden, lowering the flaps to full flaps caused the same nose down thing to happen. The PIC had the Viscount thing in his memory and instantly moved the flaps up and successfully recovered. I also believe he told me he did a second attempt with the same result. Third try without full flaps was successful.
After a long time of investigation by SAAB they found that ice formed as sandpaper on the leading edges caused this to happen. That's why the SAAB 340 leading edges on the tailplane are pointing upwards nowadays.
411A mentions the Viscount stalling problems.
In fact, the real problem was tailplane stalling when flaps were LOWERED. This action needed more negative lift from the tail which itself stalled as more rearward pressure was applied to the control column. No anti-ice, or minimal temperature resulted in a severe and immediate nose drop.</Quote>
A similar thing actually happened to the SAAB 340 during I believe the second or third year of operation. During an approach in the north of Sweden, lowering the flaps to full flaps caused the same nose down thing to happen. The PIC had the Viscount thing in his memory and instantly moved the flaps up and successfully recovered. I also believe he told me he did a second attempt with the same result. Third try without full flaps was successful.
After a long time of investigation by SAAB they found that ice formed as sandpaper on the leading edges caused this to happen. That's why the SAAB 340 leading edges on the tailplane are pointing upwards nowadays.
Last edited by Tjosan; 16th Dec 2007 at 06:42.
Join Date: Aug 2007
Location: Scotland
Posts: 41
Likes: 0
Received 0 Likes
on
0 Posts
I fly the SAAB daily in Scotland. Boots must go on early at the first sign of ice, props must go on late with air temp of -5 or below AND significant evidence of ice accretion and or prop vibration/thrust loss. SAAB is a big sufferer of run-back ice on the props causing upto 30% prop thrust reduction if not treated as per SAAB's advice. Trust me, I know. It's not a dangerous aeroplane if operated properly. Infact it's one of the most fun aeroplanes I've had the fortune to fly, in any weather.
Join Date: Jul 2000
Location: U.S.A.
Posts: 474
Likes: 0
Received 0 Likes
on
0 Posts
One of the most fortunate outcomes of a turboprop icing accident I am aware of….…..
First, here is the link to the NTSB report:
http://www.ntsb.gov/ntsb/brief.asp?e...03X00453&key=1
Now, the story behind the story…..the airplane crashed through the (flat} roof of an office building……the fuselage went between the trusses of the roof structure (wings took the load of impact), and on the way through the structure, it broke the sprinkler system pipes, which sprayed water over the site…..no fire. All survived, most with minor injuries.
There was a picture in the Reno paper of the airplane half in/half out of the roof of the building. Unlucky to crash…..incredibly lucky to survive.
First, here is the link to the NTSB report:
http://www.ntsb.gov/ntsb/brief.asp?e...03X00453&key=1
Now, the story behind the story…..the airplane crashed through the (flat} roof of an office building……the fuselage went between the trusses of the roof structure (wings took the load of impact), and on the way through the structure, it broke the sprinkler system pipes, which sprayed water over the site…..no fire. All survived, most with minor injuries.
There was a picture in the Reno paper of the airplane half in/half out of the roof of the building. Unlucky to crash…..incredibly lucky to survive.
Join Date: Aug 2006
Location: norway
Posts: 71
Likes: 0
Received 0 Likes
on
0 Posts
Turboprops and Icing.
Following the ice-related accidents in 1994 and 1997 to an ATR 72 and an Embraer 120 the FAA proposed amendments to the FAR`s.
The proposals refer in part to NTSB recommendations following the afore mentioned accidents and includes "a revision of manuals and training procedures to emphasise that leading edge de-icing boots should be activated as soon as the airplane enters icing conditions".
In addition, the NTSB stated that AFM recommendations to delay activation of the deicing boots until a specific amount of ice had accumulated should be revised.
Further, the NTSB said that the belief that premature activation of the boots might cause ice to form a bridge over the boots, rendering them ineffective, is not valid. Ice-bridging, said the NTSB, does not occur on modern aircraft, therefore the ice protection system should be activated as soon as the airplane enters icing conditions.
A detailed report of the EMB 120 accident in 1997 and the subsequent actions of the FAA can be obtained from the Flight Safety Foundation`s Accident Prevention bulletin Vol.56 No.3 of March 1999.
I seem to remember that there was some controversy attached to the implementation of the FAA`s directives; many of the turboprop operators failed to amend their AFM`s or training procedures, a deficiency which apparently was not corrected by local FAA inspectors.
The proposals refer in part to NTSB recommendations following the afore mentioned accidents and includes "a revision of manuals and training procedures to emphasise that leading edge de-icing boots should be activated as soon as the airplane enters icing conditions".
In addition, the NTSB stated that AFM recommendations to delay activation of the deicing boots until a specific amount of ice had accumulated should be revised.
Further, the NTSB said that the belief that premature activation of the boots might cause ice to form a bridge over the boots, rendering them ineffective, is not valid. Ice-bridging, said the NTSB, does not occur on modern aircraft, therefore the ice protection system should be activated as soon as the airplane enters icing conditions.
A detailed report of the EMB 120 accident in 1997 and the subsequent actions of the FAA can be obtained from the Flight Safety Foundation`s Accident Prevention bulletin Vol.56 No.3 of March 1999.
I seem to remember that there was some controversy attached to the implementation of the FAA`s directives; many of the turboprop operators failed to amend their AFM`s or training procedures, a deficiency which apparently was not corrected by local FAA inspectors.
Join Date: Jan 2006
Location: Between a rock and a hard place
Posts: 1,267
Likes: 0
Received 0 Likes
on
0 Posts
Seven Fifty Seven wrote:
I fly the SAAB daily in Scotland. Boots must go on early at the first sign of ice, props must go on late with air temp of -5 or below AND significant evidence of ice accretion and or prop vibration/thrust loss. SAAB is a big sufferer of run-back ice on the props causing upto 30% prop thrust reduction if not treated as per SAAB's advice. Trust me, I know. It's not a dangerous aeroplane if operated properly. Infact it's one of the most fun aeroplanes I've had the fortune to fly, in any weather.
There is no risk of 'ice bridging' when operating the boots at such an early stage? Never flown with boots but always taught that boots should not be operated until a certain amount of ice has built up, otherwise the boot is not able to crack the ice?
I fly the SAAB daily in Scotland. Boots must go on early at the first sign of ice, props must go on late with air temp of -5 or below AND significant evidence of ice accretion and or prop vibration/thrust loss. SAAB is a big sufferer of run-back ice on the props causing upto 30% prop thrust reduction if not treated as per SAAB's advice. Trust me, I know. It's not a dangerous aeroplane if operated properly. Infact it's one of the most fun aeroplanes I've had the fortune to fly, in any weather.
There is no risk of 'ice bridging' when operating the boots at such an early stage? Never flown with boots but always taught that boots should not be operated until a certain amount of ice has built up, otherwise the boot is not able to crack the ice?
I REALLY SHOULDN'T BE HERE
Never flown with boots but always taught that boots should not be operated until a certain amount of ice has built up, otherwise the boot is not able to crack the ice?