Shore Guy
29th Nov 2000, 02:11
Ladies and/or Gentlemen:
Some time back, we were conducting an arrival/approach in one of our B767-300ER's. We were placed on the (unexpectedly long)downwind leg for the ILS and assigned a speed of 170 Kt. At our weight, that required a flap setting of 5 (for those unfamiliar, flaps 5 in the 767 is leading edge devices fully extended, trailing edge devices partially extended). We were in icing conditions (ice detect light on and off a few times, no buildup on wiper blades). When finally put on final approach, we continued to configure the aircraft. When going from flaps 20 to flaps 30, the airframe began to vibrate considerably and the yoke began to pulse fore and aft (A/P on). We added thrust, disconnected the A/P, and increased speed to REF + 15K. Vibration reduced substantially and pulsing of yoke stopped. After discussion, we elected to continue the approach at flaps 30 at the higher speed. Approach, landing and rollout were normal. Temp on the ground was +2C.
We left the flaps down and taxied to our ramp. Inspection revealed some ice in the flap hinge area but, with the temperature above freezing, most of the "evidence" was gone. We suspected that the vibration/pulsing was caused by ice forming on the leading edge of the trailing edge devices, and when flaps 30 was selected, the flow was so disturbed over the flaps and subsequently the tail, the vibration/pulsing ensued.
The next day, I contacted our Boeing rep for any additional information. Boeing does not care to correspond with individual pilots, so all reports were channeled through our flight standards group. After some time, Boeing responded that they had received a couple of similar reports and if it happened again, to pull the FDR and submit its data.
I recently followed up on this incident and found that Boeing had received enough reports of similar occurences to conduct a flight test program and ultimately issue a Flight Operations Technical Bulletin on the subject (follows).
Their flight testing confirmed our suspicions of a disturbed flow over the TE flaps.
In a sense, we all become experimental flight test pilots when ice forms on the airframe. Not all combinations of icing types, speed, angle of attack, configuration are tested during certification. The fact that a mature program such as the 767 can demonstrate some "unknown" handling characteristics is testimony to that.
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BOEING COMMERCIAL AIRPLANE GROUP
FLIGHT OPERATIONS TECHNICAL BULLETIN
NUMBER: 767-63
DATE: December 17, 1999
This bulletin provides information, which may prove useful in airline operations or airline training. The information provided in this bulletin is not critical to flight safety. The information may not apply to all customers; specific effectivity can be determined by contacting The Boeing Company. This information will remain in effect depending on production changes, customer-originated modifications, and Service Bulletin incorporation. Information in this bulletin is supplied by the Boeing Company and may not be approved or endorsed by the FAA at the time of writing. Appropriate formal documentation will be revised, as necessary to reflect the information contained in this bulletin. For further information, contact Boeing Commercial Airplane Group, Chief Pilot -Training, Technical, and Standards, Flight Crew Operations, P.O. Box 3707, Mail Code 14-HA, Seattle, Washington 98124-2207; Phone (206) 655-1400; Facsimile (206) 655-3694; SITA: SEABO7X Station 627.
SUBJECT: Airframe Buffet on Approach in Icing Conditions
ATA NO: 30-10
APPLIES TO: All 767-300 Airplanes with Improved Flap System and All 767-300ER
Airplanes
BACKGROUND:
During the All-Operators Symposium on March 30, 1999, operators were informed that over the last few years The Boeing Company had received 15 reports from 4 operators of higher than “normal” airframe buffet occurring after extension to Flaps 30 while on approach in icing conditions. A few reports also included observation of control-column oscillations or of the Vref bug being inside the EFIS speedtape amberband. All of the reports involved 767-300ER airplanes or 767–300 airplanes with the improved flap system, which has higher flap deflections for Flaps 30 than the original 767200 and 767-300 flap systems. In late 1998, Flight Data Recorder data were provided to Boeing for analysis of three flights where buffet had been reported during approaches in icing conditions. These data showed higher-than- expected buffet levels for two of the three flights. The data for the two higher-buffet approaches also indicated that reduced maneuver capability due to lift losses at Flaps 30 could be associated with ice accretion. At the March Symposium, operators were informed that Boeing had discussed these reports with the FAA and intended to conduct flight testing to investigate and resolve this issue.
FLIGHT TEST PROGRAM:
Boeing conducted a flight test program on a new 767-300 during May of 1999. Several artificial-ice configurations were tested. The baseline configuration consisted of 3-inch artificial ice shapes installed on the horizontal tail and on the wing leading-edge sections that are not protected by the thermal anti-ice system. These ice shapes were representative of ice accretion during a Flaps Up hold and were used as part of the original 767 flight-in-icing certification. While retaining the 3-inch baseline ice shape installation, two different artificial flap ice shapes, “small flap ice” and “large flap ice,” were tested. The “small flap ice” represented a thin, frost-like ice accumulation, which was considered representative of the configuration experienced in many of the in-service reports. The “large flap ice,” a more substantial ice shape with a 1.2-inch horn, represented extended exposure to icing, such as during a flaps-down hold, and was considered a conservative configuration. The flap ice shapes were installed on the leading edges of the inboard and outboard trailing-edge flaps. Tests were conducted at both forward and aft centers of gravity (cg’s).
FLIGHT TEST PROGRAM RESULTS:
· Buffet
With the baseline 3-inch ice shapes, buffet levels increased with increasing flap deflection and were consistent with the 767-200 buffet levels with ice at equivalent flap deflection angles. The increased flap deflections at Flaps 30 with the improved 767-300 and 767-300ER flap system resulted in higher buffet levels than seen on the 767-200 at Flaps 30. The buffet increase was primarily driven by the horizontal tail and increased at more forward cg’s. Elevator and stabilizer effectiveness were thoroughly tested and found to be satisfactory.
With flap ice there was an additional buffet increase, again growing larger as flap deflection increased. The small flap ice configuration at Flaps 30 exhibited the highest buffet levels generated during the flight test program. It was similar to the buffet level for a non-iced airplane at Flaps 30 with the speedbrakes up. Flow visualization data suggested that the cause of the increased buffet was intermittent flow separation on the outboard flap upper surface. With large flap ice shapes, buffet increased at Flaps 25 and 30 although not quite as much as with the small flap ice. The flow visualization data indicated nearly complete flow separation on both the inboard and outboard flap upper surfaces at Flaps 30. The buffet levels for all ice configurations were found to be acceptable, and did not interfere with operation of the aircraft.
· Lift at Normal Operating Speeds
There was no effect on lift at normal operating speeds with the 3-inch ice shapes on the wing and empennage, nor with either flap ice shape at detents less than Flaps 25. There was a lift decrement with both small and large flap ice at Flaps 25 and small flap ice at Flaps 30, equivalent to a change in airspeed of 1 to 4 knots. Flaps 30 with the large flap ice had a somewhat more significant lift loss, equivalent to approximately 12 knots, consistent with separated flow on the flaps.
· Stall Speeds & Stall Warning Margin
The baseline 3-inch ice shapes on the wing leading edge caused an increase in stall speeds relative to the clean airplane for all flap detents except Flaps Up. Relative to this level, there was an additional increase in landing flap stall speeds with both small and large flap ice. Adequate stall warning margin was demonstrated for all configurations.
· Maneuvering Capability
There was no reduction of maneuvering capability to stall warning at Flaps Up through Flaps 20 for any of the ice configurations tested. The large flap ice resulted in a 3- to 4-degree reduction in Flaps 25 maneuver capability at a speed of Vref25, the bank angle capability for this configuration exceeding 40 degrees. With small flap ice at Flaps 30, Vref30, 40-degree maneuver capability to stall warning was demonstrated. With large flap ice at Vref30, the demonstrated maneuver capability was slightly reduced to approximately 35 degrees, consistent with the lift loss. At Vref30+5, Flaps 30 bank-angle capability in excess of 40 degrees exists for all ice configurations.
· Drag
The drag increase due to the 3-inch ice shapes was in agreement with the certified drag levels. There was no appreciable drag increase with either the large or small flap ice configurations. For all configurations, the incremental ice drag was less than or equal to the increment currently accounted for in the AFM.
· Landings
Both manual and automatic landings were performed at Flaps 25 and 30 with the conservative configuration of 3-inch baseline ice shapes plus large flap ice. Flaps 30 touchdown speeds ranged from Vref30 to Vref30+11. Adequate margin to tail skid contact was demonstrated in all cases. During some Flaps 30 approaches, the Vref bug was inside the speedtape amberband, which is consistent with the reduction in lift and maneuvering capability with the large flap ice. On average, the Vref bug was several knots inside the amberband and for brief periods a difference exceeding 10 knots was seen. With autothrottle on, approach speeds were maintained above the amberband. During automatic approaches, oscillation or “pulsing” of the control column was experienced. The oscillation appeared to be caused by excitation of the natural frequency of the control-column cable system in response to the airframe buffeting. The control-column oscillation was not driven by the autopilot and had no effect on autopilot or elevator performance. The control-column movement is considered to be a normal characteristic of the 767 under these flight conditions. There were no adverse handling characteristics noted.
SUMMARY AND CONCLUSIONS:
The FAA participated in the flight test program and the results were reviewed with representatives of both the FAA and JAA. It was agreed that there are no safety issues related to flight in icing for the 767-300 or 767-300ER. Although there are increased buffet levels associated with ice accretion on the horizontal tail and trailing edge flaps, the airplane meets all applicable buffet, aerodynamic performance, and handling quality regulations. No changes to operating procedures or AFM parameters are required.
Based upon the results of this testing, The Boeing Company is informing the operating community of the performance characteristics of the 767-300 and 767-300ER with ice accretion. This Technical Bulletin is part of that educational effort. In addition, operators will find new language specifically addressing this issue in the upcoming revision of the 757/767 Flight Crew Training Manual, as well as the next edition of the 767 Operations Manual. A copy of the new “Operation in Icing Conditions” section from the revised Flight Crew Training Manual is attached.
Some time back, we were conducting an arrival/approach in one of our B767-300ER's. We were placed on the (unexpectedly long)downwind leg for the ILS and assigned a speed of 170 Kt. At our weight, that required a flap setting of 5 (for those unfamiliar, flaps 5 in the 767 is leading edge devices fully extended, trailing edge devices partially extended). We were in icing conditions (ice detect light on and off a few times, no buildup on wiper blades). When finally put on final approach, we continued to configure the aircraft. When going from flaps 20 to flaps 30, the airframe began to vibrate considerably and the yoke began to pulse fore and aft (A/P on). We added thrust, disconnected the A/P, and increased speed to REF + 15K. Vibration reduced substantially and pulsing of yoke stopped. After discussion, we elected to continue the approach at flaps 30 at the higher speed. Approach, landing and rollout were normal. Temp on the ground was +2C.
We left the flaps down and taxied to our ramp. Inspection revealed some ice in the flap hinge area but, with the temperature above freezing, most of the "evidence" was gone. We suspected that the vibration/pulsing was caused by ice forming on the leading edge of the trailing edge devices, and when flaps 30 was selected, the flow was so disturbed over the flaps and subsequently the tail, the vibration/pulsing ensued.
The next day, I contacted our Boeing rep for any additional information. Boeing does not care to correspond with individual pilots, so all reports were channeled through our flight standards group. After some time, Boeing responded that they had received a couple of similar reports and if it happened again, to pull the FDR and submit its data.
I recently followed up on this incident and found that Boeing had received enough reports of similar occurences to conduct a flight test program and ultimately issue a Flight Operations Technical Bulletin on the subject (follows).
Their flight testing confirmed our suspicions of a disturbed flow over the TE flaps.
In a sense, we all become experimental flight test pilots when ice forms on the airframe. Not all combinations of icing types, speed, angle of attack, configuration are tested during certification. The fact that a mature program such as the 767 can demonstrate some "unknown" handling characteristics is testimony to that.
- - - - - - - - - - - - - - - - - - - - - - -
BOEING COMMERCIAL AIRPLANE GROUP
FLIGHT OPERATIONS TECHNICAL BULLETIN
NUMBER: 767-63
DATE: December 17, 1999
This bulletin provides information, which may prove useful in airline operations or airline training. The information provided in this bulletin is not critical to flight safety. The information may not apply to all customers; specific effectivity can be determined by contacting The Boeing Company. This information will remain in effect depending on production changes, customer-originated modifications, and Service Bulletin incorporation. Information in this bulletin is supplied by the Boeing Company and may not be approved or endorsed by the FAA at the time of writing. Appropriate formal documentation will be revised, as necessary to reflect the information contained in this bulletin. For further information, contact Boeing Commercial Airplane Group, Chief Pilot -Training, Technical, and Standards, Flight Crew Operations, P.O. Box 3707, Mail Code 14-HA, Seattle, Washington 98124-2207; Phone (206) 655-1400; Facsimile (206) 655-3694; SITA: SEABO7X Station 627.
SUBJECT: Airframe Buffet on Approach in Icing Conditions
ATA NO: 30-10
APPLIES TO: All 767-300 Airplanes with Improved Flap System and All 767-300ER
Airplanes
BACKGROUND:
During the All-Operators Symposium on March 30, 1999, operators were informed that over the last few years The Boeing Company had received 15 reports from 4 operators of higher than “normal” airframe buffet occurring after extension to Flaps 30 while on approach in icing conditions. A few reports also included observation of control-column oscillations or of the Vref bug being inside the EFIS speedtape amberband. All of the reports involved 767-300ER airplanes or 767–300 airplanes with the improved flap system, which has higher flap deflections for Flaps 30 than the original 767200 and 767-300 flap systems. In late 1998, Flight Data Recorder data were provided to Boeing for analysis of three flights where buffet had been reported during approaches in icing conditions. These data showed higher-than- expected buffet levels for two of the three flights. The data for the two higher-buffet approaches also indicated that reduced maneuver capability due to lift losses at Flaps 30 could be associated with ice accretion. At the March Symposium, operators were informed that Boeing had discussed these reports with the FAA and intended to conduct flight testing to investigate and resolve this issue.
FLIGHT TEST PROGRAM:
Boeing conducted a flight test program on a new 767-300 during May of 1999. Several artificial-ice configurations were tested. The baseline configuration consisted of 3-inch artificial ice shapes installed on the horizontal tail and on the wing leading-edge sections that are not protected by the thermal anti-ice system. These ice shapes were representative of ice accretion during a Flaps Up hold and were used as part of the original 767 flight-in-icing certification. While retaining the 3-inch baseline ice shape installation, two different artificial flap ice shapes, “small flap ice” and “large flap ice,” were tested. The “small flap ice” represented a thin, frost-like ice accumulation, which was considered representative of the configuration experienced in many of the in-service reports. The “large flap ice,” a more substantial ice shape with a 1.2-inch horn, represented extended exposure to icing, such as during a flaps-down hold, and was considered a conservative configuration. The flap ice shapes were installed on the leading edges of the inboard and outboard trailing-edge flaps. Tests were conducted at both forward and aft centers of gravity (cg’s).
FLIGHT TEST PROGRAM RESULTS:
· Buffet
With the baseline 3-inch ice shapes, buffet levels increased with increasing flap deflection and were consistent with the 767-200 buffet levels with ice at equivalent flap deflection angles. The increased flap deflections at Flaps 30 with the improved 767-300 and 767-300ER flap system resulted in higher buffet levels than seen on the 767-200 at Flaps 30. The buffet increase was primarily driven by the horizontal tail and increased at more forward cg’s. Elevator and stabilizer effectiveness were thoroughly tested and found to be satisfactory.
With flap ice there was an additional buffet increase, again growing larger as flap deflection increased. The small flap ice configuration at Flaps 30 exhibited the highest buffet levels generated during the flight test program. It was similar to the buffet level for a non-iced airplane at Flaps 30 with the speedbrakes up. Flow visualization data suggested that the cause of the increased buffet was intermittent flow separation on the outboard flap upper surface. With large flap ice shapes, buffet increased at Flaps 25 and 30 although not quite as much as with the small flap ice. The flow visualization data indicated nearly complete flow separation on both the inboard and outboard flap upper surfaces at Flaps 30. The buffet levels for all ice configurations were found to be acceptable, and did not interfere with operation of the aircraft.
· Lift at Normal Operating Speeds
There was no effect on lift at normal operating speeds with the 3-inch ice shapes on the wing and empennage, nor with either flap ice shape at detents less than Flaps 25. There was a lift decrement with both small and large flap ice at Flaps 25 and small flap ice at Flaps 30, equivalent to a change in airspeed of 1 to 4 knots. Flaps 30 with the large flap ice had a somewhat more significant lift loss, equivalent to approximately 12 knots, consistent with separated flow on the flaps.
· Stall Speeds & Stall Warning Margin
The baseline 3-inch ice shapes on the wing leading edge caused an increase in stall speeds relative to the clean airplane for all flap detents except Flaps Up. Relative to this level, there was an additional increase in landing flap stall speeds with both small and large flap ice. Adequate stall warning margin was demonstrated for all configurations.
· Maneuvering Capability
There was no reduction of maneuvering capability to stall warning at Flaps Up through Flaps 20 for any of the ice configurations tested. The large flap ice resulted in a 3- to 4-degree reduction in Flaps 25 maneuver capability at a speed of Vref25, the bank angle capability for this configuration exceeding 40 degrees. With small flap ice at Flaps 30, Vref30, 40-degree maneuver capability to stall warning was demonstrated. With large flap ice at Vref30, the demonstrated maneuver capability was slightly reduced to approximately 35 degrees, consistent with the lift loss. At Vref30+5, Flaps 30 bank-angle capability in excess of 40 degrees exists for all ice configurations.
· Drag
The drag increase due to the 3-inch ice shapes was in agreement with the certified drag levels. There was no appreciable drag increase with either the large or small flap ice configurations. For all configurations, the incremental ice drag was less than or equal to the increment currently accounted for in the AFM.
· Landings
Both manual and automatic landings were performed at Flaps 25 and 30 with the conservative configuration of 3-inch baseline ice shapes plus large flap ice. Flaps 30 touchdown speeds ranged from Vref30 to Vref30+11. Adequate margin to tail skid contact was demonstrated in all cases. During some Flaps 30 approaches, the Vref bug was inside the speedtape amberband, which is consistent with the reduction in lift and maneuvering capability with the large flap ice. On average, the Vref bug was several knots inside the amberband and for brief periods a difference exceeding 10 knots was seen. With autothrottle on, approach speeds were maintained above the amberband. During automatic approaches, oscillation or “pulsing” of the control column was experienced. The oscillation appeared to be caused by excitation of the natural frequency of the control-column cable system in response to the airframe buffeting. The control-column oscillation was not driven by the autopilot and had no effect on autopilot or elevator performance. The control-column movement is considered to be a normal characteristic of the 767 under these flight conditions. There were no adverse handling characteristics noted.
SUMMARY AND CONCLUSIONS:
The FAA participated in the flight test program and the results were reviewed with representatives of both the FAA and JAA. It was agreed that there are no safety issues related to flight in icing for the 767-300 or 767-300ER. Although there are increased buffet levels associated with ice accretion on the horizontal tail and trailing edge flaps, the airplane meets all applicable buffet, aerodynamic performance, and handling quality regulations. No changes to operating procedures or AFM parameters are required.
Based upon the results of this testing, The Boeing Company is informing the operating community of the performance characteristics of the 767-300 and 767-300ER with ice accretion. This Technical Bulletin is part of that educational effort. In addition, operators will find new language specifically addressing this issue in the upcoming revision of the 757/767 Flight Crew Training Manual, as well as the next edition of the 767 Operations Manual. A copy of the new “Operation in Icing Conditions” section from the revised Flight Crew Training Manual is attached.