When Windshields Fail Business & Commercial Aviation Patrick R. Veillette: aviationweek Absolute Aviation English. A rare event that demands calm while managing complications Consider for a moment your aircraft’s windshield and the stresses it endures as a matter of course. It might bake to 130F on a heat-soaked ramp and 20 min. later be pushing through the stratosphere at 500 kt. in air super cooled to -45F. Moreover, it holds firm despite the pressure differential. We take all of that for granted since it’s designed to withstand those extremes. But what if it fails? It happens. Fortunately, the majority of failures aren’t as catastrophic as that encountered by Capt. Tim Lancaster in his BAC 1-11 (see sidebar), but there’s nothing benign about any of them as the flight crew of a Cessna Citation X discovered during a flight from Samoa to Sydney on Jan. 15, 2013. Shortly after leveling at FL 450, the pilots observed this EICAS message: WSHLD HEAT INOP L (windshield heat inoperative left). The checklist said to leave icing conditions as soon as practical. Approximately 2 min. later, the left windshield’s outer ply shattered with a loud bang. The crew donned their oxygen masks, commenced an immediate descent along with a turn toward Nadi International Airport at Fiji, deployed the passenger oxygen masks, and declared MAYDAY to Nadi Radio. Once this was established, the pilot not flying (PNF) ensured the passengers were on oxygen and briefed them on the situation, which included preparing for a possible ditching and donning life vests. The pilot flying (PF) reported that the windshield crack remained constant and did not grow any more, and much to their relief the cabin did not depressurize. The crew subsequently downgraded to a PAN and landed at Nadi without further incident. There were no injuries. Subsequent examination of the electrically heated glass left windshield by the Australian Transport Safety Board (ATSB) found that the outer ply had shattered across the entire surface. There was brown discoloration near the top right of the windshield with staining around one of the soldered contacts. At the time of the failure the windshield had accumulated approximately 3,200 flight hours and 2,050 flight cycles. (See photos on next page.) Windshields are designed under the “fail safe” concept, which basically means that the failure of one component won’t lead to a catastrophic failure of the structure. Accordingly, it’s common for windshields on jets to be constructed of multiple layers to withstand the immense thermal, aerodynamic and mechanical stresses. For example, the Cessna 750’s windshield construction consists of a 0.10-in. outer (non-structural) face ply, a middle 0.19-in. structural ply and a 0.235-in. structural inner ply, separated by 0.15-in. PVB/urethane layers for a total thickness of 0.825 in. Either the middle or inner pane is structurally capable of maintaining cabin pressure. The interlayer between the outer and middle glass panes is heated. Failure analysts determined that wear of the seal at the top of the windshield allowed moisture ingress to the bus bar and led to degradation of the electrical connection between the bus bar and the heating film. Eventually the heating film began to burn out, in turn leading to arcing and failure of the outer non-structural face ply of glass. The manufacturer addressed this issue by adding a fiberglass “z-shaped” strap along the boundary of the face ply to provide an additional layer for moisture and for an indication of seal wear. Investigators also found that the cracking could occur in the solder material, resulting in a short circuit and arcing. This issue was addressed by changing the size and location of the upper bus bar in the window. The ATSB’s report on the Citation windshield incident aptly demonstrates many of the complicated factors facing pilots when a windshield cracks. The report concluded “precautions taken by the flight crew to descend to a lower altitude and diversion to the alternate airport highlighted the importance of good flight planning.” How often and how severe are windshield failures? A review of the U.K. Civil Aviation Authority’s Mandatory Occurrence Reporting system for incidents involving failure or malfunction of a pilot’s windscreen found a total of 195 incident reports involving windshield failures over a five-year period (April 2008 — April 2013). The aircraft involved ranged from light planes and helicopters to turboprops and airliners. Of the total, 171 were fixed-wing airplanes, and 24 were helicopters. Focusing on the airplanes, the data revealed that the structural integrity of multi-layered windshields common to FAR Part 25 aircraft was always maintained, a tribute to proper design, manufacturing and maintenance procedures. Nonetheless, the events often involved obscured vision, arcing and the high workload associated with taking corrective procedures including rapid descent as prescribed by the abnormal section of the checklist, along with the additional flight planning for a divert. General aviation aircraft certified under Part 23 were involved in four of the windshield failures, all of which were caused by bird strikes. In two cases, remnants of the bird violently struck the pilot, causing significant head injuries, even knocking off a headset, and sending bloody debris all over the cabin. These pilots were able to recover, albeit with difficulty. The phase of flight at which the failure occurs has a large effect on the immediate action required of the pilot. For jet and multiengine-turboprop aircraft, around three-quarters of the incidents (74%) happened during high-altitude cruise, a period when the windshield is subjected to extremes in aerodynamic force, thermal stress and pressure differential. The balance of jet and multiengine-turboprop aircraft incidents occurred on approach and landing, climb and descent. Almost all the incidents (98%) involved a significantly shattered or cracked layer of the windscreen. One-quarter (25%) of them exhibited electrical arcing after the windscreen shattered, while nearly one in 10 (9%) involved arcing prior to the shattering. Bird strikes were the initiating event in 12 of the fixed-wing incidents, while hail damage caused three. Once the cracks occurred, about one-third (36%) of the crews began descending immediately and 14% donned their oxygen masks in anticipation of loss of pressurization. Most of the incidents (86%) involved significant immediate action, and those that did not (14%) occurred while on approach with the pilots continuing the approach to a landing. Nearly one-fifth (17%) of the incidents were preceded by a precautionary warning of a windshield heat failure, followed shortly thereafter by the abrupt shatter of one of the panes within the windshield. A windshield must withstand extreme temperature differences in a short time frame, which subjects it to formidable thermal stresses as it attempts to expand or contract. There is a conductive layer between the plies that heats the window unit. In some windows you will notice an electrical grid in the corner of the windshield, which is the electrical connection from the power source. Within the windshield system there are thermostats to keep the transparencies from overheating, but these can fail. Should that happen in flight, there’s an airspeed restriction for many aircraft, which is published in their respective AFMs. If the heating is inadvertently left on during ground operations with no cooling air flow over the windshield, it can be prone to overheating, which can result in a peculiar wavy distortion when looking through the windshield. Changes in temperature also cause large differences in a window’s elasticity. This is important because a cold-soaked window is brittle and less able to resist an impact (namely from a bird), whereas a warm window would be more resistant to impact damage. In the Boeing 777, window heat turns on automatically at the beginning of a flight and off at the end, a feature intended to lower pilot workload, allowing the pilots to concentrate on the flight trajectory and configuration management tasks rather than these tedious “housekeeping” tasks. In most other aircraft this is done manually and is sometimes overlooked with potentially negative consequences. In addition to being turned on, proper operation of the windshield heating system is dependent upon an electrical supply, and electrical system failures such as a total generator failure can shed the windshield heat circuit, as well as other circuits not deemed absolutely vital for the continuation of the flight. For instance, the flight crew of an Airbus 321 on Jan. 12, 2009, experienced a generator failure, which led to numerous electronic centralized aircraft monitor (ECAM) failure messages and loss of the flight control unit (FCU) and pedestal. The pilots faced a cascading series of electrical failures that included the air-conditioning, pressurization, windshield heat, flight management system, landing gear and central warning systems. Ice and mist accumulated on the windshield, which made the landing approach quite challenging, not to mention post-touchdown challenges posed by the lack of autobraking and nosewheel steering. Despite all of that trouble, the cold-soaked pilots were able bring the aircraft safely to a stop. A loud, high-pitched squeal emanating from the vicinity of a window seal was reported in 12% of the incidents. On Oct. 31, 2009, a Boeing 757’s flight crew heard such a sound coming from the area of the captain’s window. The pilots were understandably worried about loss of pressurization and diverted the flight for a precautionary landing. A maintenance review found that during the preceding flight, the crew had experienced a similar noise, but that had ceased after approximately 30 min. After that flight, the window seal had been inspected, but no damage was found. The window seal was then cleaned and lubricated, and the aircraft released. Following the incident flight, the seal was found to be excessively greased and the inflation holes were blocked. Following a pressure test, leaks were found in the No. 3 left-hand window due to the lower edge blanking screws being loose. If you have ever written up an aircraft for a suspected seal failure and afterward the maintenance technicians “could not duplicate” the noise, don’t be surprised. Window seal failures can be difficult to diagnose on the ground and it requires a lot of effort and proper equipment to conduct a pressure test. It is worth taking a close look at the guidance in your flight manual for the proper procedures following a windshield failure, and perhaps asking for a bit of extra clarification the next time you attend simulator training. These are procedures you don’t want to read for the first time at altitude with the howls emanating from the glass just a few feet in front of your face. The “Operating Limits” section of the Citation X’s flight manual states, “If either cockpit windshield cracks in flight, continued flight to destination is permitted in accordance with Section III, Abnormal Procedures, Cockpit Forward or Side Windshield Cracked or Shattered.” It might surprise you to learn that after landing, if only the Citation X’s outer (non-structural) ply of the windshield is cracked, flight to a maintenance base is permitted, observing Cockpit Forward or Side Windshield Cracked or Shattered Procedures. If either structural ply of the windshield is cracked, however, restricted flight is permitted only on a ferry permit (Special Airworthiness Certificate). Speed, altitude and cabin pressurization reductions are common specified procedures in aircraft flight manuals for a compromised windshield. For example, the King Air 200’s flight manual Abnormal Procedures section for a Cracked or Shattered Windshield prescribes maintaining an altitude of 25,000 ft. or less, resetting the pressurization controller to maintain a cabin differential pressure of 2.0 to 4.6 psi, and then depressurizing the cabin prior to touchdown. Review of the 195 windshield failures revealed that there were often other complicating factors that made it more difficult for the flight crew to get the aircraft safely on the ground. Visibility through a shattered windshield could be sufficiently reduced to dictate flying the airplane from the opposite side of the cockpit. A shattered inner ply can cause particles or flakes that could interfere with a crew’s vision. Windshield heat may be inoperative in the area of the crack(s). A cracked outer windshield ply may damage operating windshield wipers. In addition, flight planning can quickly consume the crew’s attention as a result of having to cruise at restricted (lower) flight levels, and perhaps needing to dump fuel to get down to landing weight if an immediate landing is warranted. The situation can be further complicated in international (non-radar) airspace in which a precautionary descent and/or turn to a diversionary airport is initiated, causing a possible procedural loss of airspace separation. In short, while windshield failure is rare, the potential complexity of dealing with such a dangerous condition merits consideration and planning and is certainly worth discussion during recurrent training. Absolute Aviation English - Congonhas Airport Classes at the fully structured school and online! ICAO - Bandeira - Speeches - Phraseology - Interviews (Emirates/Qatar/FlyDubai) - General - Translations (5511)3253-4367 / 5096-1618 Skype: absolute.aviation. CGH and Skype
Posted on: Fri, 29 Aug 2014 16:54:32 +0000
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