Preflight Descriptions: More Than Merely ‘Check,’ Part 1

Windshield seal
When an aircraft's exterior windshield seal deteriorates due to erosion by wind and rain, moisture seeps into the window assembly.
Credit: Patrick Veillette

Preflight descriptions should better explain “how, what and why” to properly evaluate a component’s airworthiness

The authors of preflight checklists are addicted to the “Windshield-check” or “Radome-check” format. The standard response of “check” does little to provide specific information that pilots should look for during a thorough preflight. With no more instruction than a response of “check,” a pilot might think that the examination of the windshield is merely to make certain it doesn’t look cracked.

In contrast, maintenance technicians and inspectors keep a trained eyed on important items. They also know that catching a small crack or vibration in its earliest stages can prevent worse damage from occurring.

By no means are the following examples meant to be a thorough compilation. They illustrate how “less than thorough” preflight inspections have led to inflight emergencies or required expensive corrective maintenance that could have been prevented by a timely write-up.

Windshield Seals and Delamination
A Cessna Citation 750 was en route from Samoa to Sydney on Jan. 15, 2013. Shortly after leveling at FL450, the flight crew observed a WSHLD HEAT INOP L (windshield heat inoperative left) message on the EICAS. The appropriate checklist directed the crew 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 (NAN/NFFN) at Fiji, deployed the passenger oxygen masks, and declared MAYDAY to Nadi Radio. Once this was established, the pilot not flying 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 reported that the windshield crack remained constant and did not grow any further, and much to their relief the cabin did not depressurize. The flight crew subsequently downgraded to a Pan-Pan and landed at Nadi without further incident. There were no injuries to the passengers or crew.

Subsequent examination of the left, electrically heated glass windshield by the Australian Transport Safety Bureau (ATSB) found that the outer ply had shattered across the entire surface. Brown discoloration was observed toward 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.

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. Thus, it is 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. Both the middle and inner panes are 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 moisture protection layer as well as an indication of seal wear. Investigators also found that that 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 “Windshield Cracking Event Involving Cessna Aircraft Company 750, VH-RCA” incident report aptly demonstrates many of the complicated factors facing a flight crew when a windshield cracks. The report concluded that “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.”

This wasn’t an isolated incident involving a windshield seal.

  • On May 31, 2012, during cruise flight at FL220, the LH flight deck windscreen cracked in a Cessna 560. Worried about failure of the window, the flight crew made an immediate descent for diversion. Engineering inspection identified a damaged seal that had let water in and then frozen during flight (British report 201206505). Crews were warned to pay attention to seal condition during preflight checks.
  • On Oct. 27, 2008, a Boeing 737 was climbing when it encountered pressurization warnings and a loud vibration noise from a recently replaced windscreen. The aircraft descended and returned to the departure airport. The Civil Aviation Authority investigation revealed that there had been insufficient time for the sealant to cure on the eyebrow windows, which caused them to leak (British report 200811747).
  • On May 5, 2008, the flight crew of a DHC8 heard a loud bang behind the first officer during approach. The cabin pressure indicator showed a rate of descent of approximately 2,000 fpm. A section of the weather seal was observed to have blown back from the rear of the copilot’s window. A normal approach and landing was carried out (British report 200804390).

Windshield seals are part of the pressure vessel of the aircraft. The ability of the aircraft to maintain pressure is measured and inspected during a variety of maintenance activities, schedules, etc. When an aircraft’s exterior windshield seal (often called an “aerodynamic seal”) deteriorates due to erosion by wind and rain, moisture seeps into the window assembly, causing problems such as window cracking. Proper inspection and maintenance of the aerodynamic seal are critical to prevent moisture ingress, which in turn directly contributes to extending windshield service life.

Each aircraft’s service and repair manual will contain the recommended visual inspection of the seal for erosion, cuts, nicks and overall condition. Other abnormalities such as cloudy areas in corners of a window or around the window periphery in the interlayer indicate moisture ingress should be brought to the attention of the aircraft’s maintenance technician. Burn marks, bubbles and moisture stains are other indications that a window may be reaching an unserviceable condition.

Hopefully the next time you preflight your windshield, your eye will be better informed of specific items to examine rather than the checklist’s cursory “windshield-check.”

Nosewheel Shimmy
Have you ignored the nosewheel shimmy because “It happens often and thus must be OK?” Nosewheel shimmy should not be ignored. A quick search of proposed Airworthiness Directives (AD) reveals why. For example, Quest Aircraft notified users of its Kodiak 100 that these aircraft were experiencing fatigue cracks in the nose landing gear. In one report, the nose landing gear fork failed during landing. 

On unimproved surfaces, the nose landing gear shimmy-damper system can wear and loosen, reducing the resistance of the nose gear to shimmy. The nose landing fork was not designed for the side loads caused by shimmying, which could result in fatigue cracks. This condition, if not corrected, could cause separation of the fork with consequent reduced control on landing. If the fork separates on an unimproved surface, the risk of the nose landing gear digging in and the airplane overturning on the ground increases. (Reference: Airworthiness Directives, proposed March 8, 2018, Docket No. FAA-2018-0180.)

The AD required an inspection of the shimmy damper bracket for looseness. If a loose shimmy damper bracket is found during any inspection required by the AD, rework it following the field servicing instructions. If any other damaged (loose, leaking, corroded, worn, etc.) components are found in the shimmy damper system during any inspection required by the AD, replace the damaged components before further flight.

Other components in the nose gear assembly can cause shimmy. The Citation Mustang Technical Review recommends checking the nosewheel balance and inspecting for loose or slack components. Check lubrication of the thrust bearing, which may provide temporary relief. If not, re-service the shimmy damper or re-build the nose gear. Quarterly maintenance should include lubrication of the thrust bearing.

In Part 2, we’ll discuss a few more examples of how to properly evaluate a component’s airworthiness.