Cessna Citation Excel Controls Freeze

On final approach to Traverse City, MI on 26 March 2014 the Pilot Flying of a Cessna Citation Excel N580QS discovered that the aircraft rudder pedals were ‘hard or frozen’.  The results of the subsequent investigation published by the National Transportation Safety Board (NTSB) revealed an unusual dormant failure.

History of the Flight

After a night stop in Florida on 25 March 2014, the aircraft departed for MI with 2 pilots and 2 passengers aboard on a Part 91 Subpart K: Fractional flight which was:

…uneventful until landing. The auto-pilot and yaw damper were turned off on final approach and the flying pilot (right seat pilot) noted the rudder pedals were hard or frozen during the landing. He was able to make an uneventful landing. Once safely on the runway, the flight controls were passed to the non-flying pilot (left seat pilot) to confirm the rudder pedal situation. The non-flying pilot confirmed the hard or frozen pedals. Once slowed to a safe speed, the rudder pedals were freed enough for the crew to be able to taxi to the ramp without further issues or assistance. After parking, the rudder still felt stiff and required more input that normal to operate. Once the airplane was powered down, the rudders pedals still had the same stiffness that was noted during the taxi.

Cessna Excel N580QS (Credi: via NTSB)

Cessna Citation Excel N580QS (Credi: via NTSB)

Investigation

On examination:

Water was observed draining from the black foam wrap around the [galley] drain tube’s heating element, and the black foam exhibited deformation consistent with the expansion of frozen water.

The Insulated Drain Tube (Credit: via NTSB)

The Insulated Drain Tube (Credit: via NTSB)

Removal of the foam revealed an opening in the drain tube.

Hole in the Drain Tube (Credit: via NTSB)

Hole in the Drain Tube (Credit: via NTSB)

Testing of the galley drain heating element revealed that it was inoperative. Black foam deposits were observed on a rudder cable, but no rudder system anomalies were detected.

The Drain 'Unwrapped' Showing Proximity to Flying Control Cables (Credit: via NTSB)

The Drain ‘Unwrapped’ Showing Proximity to Flying Control Cables (Credit: via NTSB)

Laboratory tests revealed that an opening in the drain tube caused the rudder cables to freeze to the drain mast, and some additional force above a baseline was needed to break the simulated rudder pedals free from a static position. At the time of the incident, the airplane’s approved inspection program did not specify that the galley drain tube or heater should be inspected. Additionally, the failure of the galley drain heater is not annunciated to the flight crew nor to maintenance personnel.

The NTSB determined the Probable Cause of this incident to be:

The inoperative galley drain tube heater, which allowed water within the tube to freeze and led to its failure; the tube’s failure allowed water to drain onto the rudder cables and freeze and resulted in the rudder binding during landing.

Action and Previous Incidents

The operator, Netjets, checked their fleet of Citation Excels in the two weeks following this event. Three more aircraft, N654QS, N698QS, and N577QS, had deformed galley drain tube insulation indicating previous freezing water damage.

Deformed Insulation on Other Aircraft in the Fleet (Credit: via NTSB)

Deformed Insulation on Other Aircraft in the Fleet (Credit: via NTSB)

The operator reviewed their usage rate in reference to the drain tube and its heater. The operator reported that in 2012 the drain tube heater, part number 105881, was replaced 12 times, in 2013 it was replaced 13 times, and in 2014 it was replaced 41 times. In 2012, the left hand weld assembly(drain tube), part number 6619292-7, was replaced 12 times, in 2013 it was replaced 8 times, and in 2014 it was replaced 18 times.

They subsequently stated to the NTSB:

As indicated in your report, failure of the heating element is not annunciated.  We are concerned with the effects undetected water and ice introduced into the galley drain area may have. As such, we believe the manufacturer should modify the aircraft inspection program to include an appropriate inspection/test of the galley drain heater and mast at a reasonable interval to ensure that they function as designed. The inspection/test may require addition of an indicator to notify crews or maintenance of heater failure.

The NTSB unfortunately have not chosen to make a recommendation and perhaps oddly report that:

…the manufacturer is designing an amendment to their maintenance manual which will include steps to engage and disengage the galley drain heater during maintenance and will include an operational test in the inspection requirements. The amendment is scheduled to be implemented in July 2015.

This footnote may partly relate to a completely different problem.  The Federal Aviation Administration (FAA) had previously issued a Special Airworthiness Information Bulletins (SAIB) CE-10-49 (first in September 2010 and then updating it in January 2011) on exactly the same part number (p/n 105881) on a range of Cessna Citations.  The SAIB dealt with a problem of the drain mast overheating during prolonged ground operation and catching fire.  The SAIB suggests that Cessna had planned to do the immanent Maintenance Manual change 5 years earlier.  While the action to solve one problem (overheating), may allow testing for dormant failures, it does introduce the risk of omitting to reconnect the heater.

Other Resources

Icing conditions (ground and in flight) was the topic for a European Aviation Safety Agency (EASAconference in 2013.

Aerossurance has also published other icing articles:

Aerossurance has extensive air safety, operations, airworthiness and safety analysis experience.  For expert business aviation support you can trust, contact: enquiries@aerossurance.com