| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 1 May 2021, at about 1200 Eastern Standard Time,[1] a Raytheon B200 aircraft was being prepared for a flight from Brisbane to Rockhampton, Queensland. The forecast weather indicated light showers of rain; however, prior to departure, heavy rain showers passed over the airport.
Upon reaching the planned cruise level of flight level[2] 260, the pilot discovered that the rudder trim was ‘frozen solid’ and could not be manipulated. The outside air temperature was -25 °C. Suspecting icing, the pilot commenced a descent and noted progressive improvement in the operation of the rudder trim once the aircraft descended into positive temperatures.
The pilot reported that normal trim operation resumed during the approach and landing. A post-flight inspection was conducted and no defects were found with the trim system.
Icing history
B200 aircraft have a known history of elevator trim icing. This has generally been found to be attributable to water freezing in the trim actuator gears and bushes, which were then redesigned to minimise the effects of ice accretion with a design that allowed the gears to crush formed ice.
Water freezing in the hinges was a problem first raised in 1981 in King Air Communique 57, and later Communique 98-002 identified piano hinges as an area prone to icing. This type of hinge is also used in the rudder trim tabs (Figure 1). The solution was to incorporate a regular lubrication schedule with a suitable grease that prevented water penetration of the hinges. Communique 90-002 included:
We have found that pressure washing of the elevator trim tab hinge washes out the lubricant, allowing moisture to enter the hingeline lugs and freeze at altitude. Even if you don't pressure wash, effects of the environment can cause the lubricant to dissipate over time and be replaced by water during foggy, misty, or rainy conditions. It is very important to re-apply the lubricant on a regular basis and lubrication schedules will vary according to the environment in which the aircraft is operated.
Both the elevator trim and rudder trim hinges have a standard lubrication interval of 200 hours, which the operator was complying with. The operator experienced a similar occurrence earlier in the year in a different B200 aircraft. It advised that although the incident aircraft were within the recommended lubrication interval, both occurrences of suspected icing followed periods of heavy rainfall on the ground prior to take-off.
Figure 1: B200 Rudder trim tab hinge location
Source: Textron Illustrated Parts Catalogue, annotated by the ATSB
Previous similar occurrences
In the 10 years prior to this occurrence, the ATSB received 48 airframe icing notifications, 22 of which involved icing of the flight controls. Aircraft that are part of the King Air series[3] accounted for approximately 15 per cent of total airframe icing reports but over a quarter (27 per cent) of all incidents that involved the icing of control surfaces. There were no records related to icing in the Civil Aviation Safety Authority defect register for the same period.
Two thirds of the aircraft that encountered control icing were regional airline turboprops and utility aircraft (such as the King Air) that typically operate at flight levels. Table 1 shows the aircraft that have reported icing control events since 2011.
Table 1: Control icing events by aircraft type
| Aircraft manufacturer | Aircraft model | Number of occurrences |
| De Havilland Canada | DHC-8 | 4 |
| Raytheon Aircraft Company | B300 | 3 |
| Raytheon Aircraft Company | B200 | 2 |
| Raytheon Aircraft Company | C90 | 1 |
| The Boeing Company | 737 | 2 |
| Israel Aircraft Industries | II-1124 | 2 |
| Learjet | L36 | 1 |
Operational exposure
The Bureau of Meteorology has produced educational material on the hazards associated with airframe icing, which stated that icing conditions are only present in temperatures between 0 ºC and -40 ºC, with the highest risk occurring between 0 ºC and -20 ºC. These temperatures occur at the flight levels where turboprop aircraft typically operate (Figure 2).
Figure 2: The icing environment
Source: Bureau of Meteorology
Tail icing
Ice accumulation on the tail is not uncommon in known icing conditions[4] and many light turboprop aircraft such as the B200 are fitted with de-ice equipment. This is typically installed on the leading edge of the wing or horizontal stabiliser but does not afford any useful protection to trim tabs located at the rear of the aerodynamic surface. Icing is often visible on the wings or windscreen, but tail icing is harder to diagnose due to the lack of visibility the pilot has of the tail section from the cockpit. As part of NASA’s in-flight icing research program, an Aircraft Icing Training course was developed that included early indicators and recovery techniques from ice-contamination tail stalls. Although this training focussed on the hazards of aerodynamic interruption due to ice accumulation, it did not refer to trim icing.
Safety action
The operator contacted the manufacturer’s field representative to confirm the correct grade of lubricant was being used and the maintenance schedule was in line with the manufacturer’s recommendations. Information about lubrication is contained in chapter 12 SERVICING – LUBRICATION SCHEDULE of the manufacturer’s maintenance manual.
Safety message
Without post-flight evidence of a defect, icing of control surfaces could go unreported. The ATSB encourages all pilots to report significant icing incidents to improve understanding of the impact icing-related occurrences may have on flight safety. This incident highlights the importance of operators tailoring service schedules to suit the environment in which the aircraft are operating.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
__________
- Eastern Standard Time (EST): Coordinated Universal Time (UTC) + 10 hours.
- Flight level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level. (FL). FL 370 equates to 37, 000 ft.
- King Air Series: Produced by Beechcraft (and now Raytheon), the King Air family of aircraft is comprised of various twin-engine turboprop variants including the B200.
- Icing conditions: Icing conditions typically exist in flight when the static outside air temperature is 5 ºC or below, and visible moisture (clouds, fog, rain, snow or sleet) are present.