Pilots confronting smoke and burning odor (SBO), a frequent reason for aircraft diversions, gain a decision-making advantage if electric cooling fans can be ruled out as the source, a U.S. manufacturer of cooling fan monitors has concluded. The advantage derives from vibration-sensing technology introduced about eight years ago to automatically detect symptoms of incipient fan failure and shut down the fan to prevent an SBO event in the first place, says Joseph Barclay, president and CEO of IWS Predictive Technologies.
Measuring temperature — the long-established method of protecting fans from a variety of hazardous conditions — has not proved effective in detecting the worn, rough, inadequately lubricated or “dry” fan impeller bearings responsible for a high proportion of all SBO events reported by airlines, Barclay said. His company basically adds a new type of predictive sensor that supplements conventional means of protecting the fans with thermal sensor–relay switches embedded in motor windings.
The typical cooling fan has a rotating impeller with bearings, rotating blades and blade-tip design that will allow interference between the blade tips and fan housing/frame if the condition of bearings deteriorates sufficiently. Company documents summarize the root cause: “Fan bearings fail often as a result of lubricant loss from high stress or high temperature conditions. Ultimately, this can result in bearing cage failure. Cage failure allows unbalanced rotation of the fan, causing fan blade tips or rotors to rub against the fan fixed assemblies [housing/frame]. This occurs at high speeds — some fans rotate in excess of 12,000 rpm — allowing friction to produce smoke and odor.” The fan continuously feeds smoke/odor into the cabin or flight deck without a failure warning or indication of cause to the flight crew.
The surprisingly excessive/premature wear of airplane fan bearings has been attributed to inherent bearing-design limitations, cost-related constraints, variation in fan re-build procedures and operating conditions of vibration, physical shock and temperature. “There had been no way to predict when any cooling fan was going to fail based on historical removal data,” Barclay said. Scheduling overhaul intervals based on average bearing service life arguably has prevented some SBO events; nevertheless, fan failures continue while other fans are unnecessarily overhauled with years of service actually left in them.
The IWS Predictive Technologies solution requires mounting a vibration monitoring unit (VMU) on the top or side of the fan housing using a bracket, vibration-transmitting feet and band-type clamps, and external cables for power and communication with the aircraft environmental control system. A VMU measures about 2 by 2 by 5 in (5 by 5 by 13 cm), and the largest eligible aircraft have eight or nine cooling fans.
The VMU comprises a solid-state accelerometer; printed circuit board with microprocessor, memory and proprietary software algorithms/profiles for real-time analysis of vibration signatures, balance and other parameters; the wired interface to the ECS and flight deck; a control relay; and light-emitting diodes that display operational and self-diagnostic information. Each unit’s vibration-signature threshold is designed to be reached before bearing deterioration can cause the interference/friction that produces smoke or a burning odor.
The applicable supplemental type certificates (STCs) today allow VMU integration with specific types of cooling fans aboard the Boeing 747-400, 757-200/300, 767-200/300/400 and 777-200/300 series. They are fitted, per regulator approvals, to fans used for avionics equipment supply and exhaust; upper/lower cabin air recirculation; gasper; galley chiller boost/exhaust; and crew rest area ventilation. The company’s first application for Airbus-related STCs, currently in the final approval stages, is expected to allow VMUs for avionics fans on the A318/A319/A320/A321 aircraft series.
As of March 2011, 12 airlines operate airplanes equipped with about 3,500 VMUs. “Before VMUs were added, one of our customers averaged one SBO event every 12,000 flights from fans,” Barclay said. “Since they began installing these monitors in 2003, we know that among the equipped aircraft, their equipped fans have completed more than 1 million flights without a single SBO event.”
He therefore expects flight crews to rely on this technology to rule out their VMU-equipped cooling fans as a smoke source as part of making timely decisions. “The flight crew can know 100 percent that the SBO is not a fan-induced event,” he said. “Smoke from the fan itself is harmless, but when the flight crew is flying an aircraft without VMUs and does not know where the smoke/burning odor is coming from, they are left trying to troubleshoot the system” — ideally following a checklist that conforms to the latest industrywide consensus on pilot responses to smoke/fire/fumes.1
VMU logic also can identify some safety-critical failures at the aircraft system level, revealing information likely to be missed by routine maintenance checks, Barclay said. The worst-case scenario is a malfunctioning fan that overheats in flight but continues running until a “catastrophic, critical meltdown-type condition” and an in-flight fire, he said.2
Diagnostics provided by the VMU after fan shutdowns also help ensure that the unit’s programmed relays function properly to preclude unwarranted fan shutdowns. Few shutdowns triggered solely by a VMU fault have occurred, however, he said.
Theoretically, substituting precision bearings for conventional bearing types would prolong service life and may help to reduce SBO events, but the price of such improvements has been considered too steep for this application, Barclay said. In response to an air-turnback SBO incident at Aberdeen, Scotland, in October 2005, the supplier of the faulty avionics vent fan replaced commonly used steel ball bearings with more costly ceramic ball bearings, for example, said the U.K. Air Accidents Investigation Branch (AAIB).3
“This approach would be good for improving reliability, but SBO is not a reliability issue — it is a failure-mode issue,” Barclay said. “Many fans already have great reliability but will still fail, and when they do, the results are the same. It’s not when they fail but how they fail.”
So far, senior airline executives have relied on varied management philosophies, safety-case methodology and cost-benefit analyses when selecting which airplanes and which specific fans will be equipped with VMUs. “We try to convince their engineers that installing VMUs across the entire fleet is the best option,” he said.
From 2002–2008, incremental improvements were made to the first VMU version. Feedback during a customer conference in 2008, as part of a company reorganization initiative during a change of company ownership and management, then led to a nine-month program of augmenting or refining capabilities, and fixing problems. “Airlines identified desired functionality changes including requests that the VMU provide the previously mentioned maintenance test of the ECS,” Barclay said.
The second version answers requests for a more convenient method of VMU and system resets after a fan replacement, and addresses several failures of ECSs to respond correctly to fan-overheat conditions detected by thermal sensors. The relevant service letter in June 2009 also details a manual bypass mode that flight crews can use to override any shutdown triggered by VMU failure; and improved diagnostics and data display modes.
In January 2011, the U.S. Federal Aviation Administration (FAA) approved the incorporation of a company-requested alternative method of compliance (AMOC) into its July 2009 airworthiness directive (AD) concerning the right recirculation fan of 757-200 series airplanes. The AD in part required detailed inspections for damage of the wire bundle, repairing any damage before further flight, rerouting the wire bundle and reorienting the fan’s electrical connector.
The AMOC allows use of IWS Predictive Technologies’ technical specifications for this inspection, repair, rerouting of the wire bundle and reorientation of the connector, which are compatible with attaching a VMU to this fan, according to Barclay. The FAA said that the AD was prompted by a report that “during landing of a Model 757 airplane, an overheat warning and smoke occurred in the main cabin, and the right recirculation fan stopped operating.”
- Flight Safety Foundation Editorial Staff. “Flight Crew Procedures Streamlined for Smoke/Fire/Fumes.” Flight Safety Digest. Volume 24 (June 2005), p. 31.
- After takeoff from Sydney, New South Wales, Australia, the flight crew of a B-747-400 returned for an uneventful landing after a fire warning message concerning the forward cargo compartment and smoke in the cabin, said the Australian Transport Safety Bureau. Investigators of the Dec. 3, 2003, SBO incident focused largely on a burned forward-galley chiller-boost fan. They found total failure of the molded-resin fan impeller blades resulting from overload fatigue. This failure was caused by worn bearings and an unbalanced rotation load, allowing blade-tip contact with the aluminum-alloy fan housing.
- AAIB Bulletin 2/2006.