“The Mystery Of The Shattering Windshields”
May 12, 2009
Articles on a episode in February 2007 during which airplane windshields were mysteriously cracking while sitting on DIA runways or in flight. Later the NTSB attributed it to sand-like debris. Others are not so sure. One airplane expert told ABC news the cracking was “not only unusual, I know of no precedent for anything like this where multiple windshields have been cracked simply by being in a particular place at a particular time.”
“We are not discounting anything,” said Federal Aviation Administration spokesman Allen Kenitzer, whose agency is among those investigating the incidents. “We believe it is very remarkable to have this number of events in the same area at the same time.”
The National Transportation Safety Board will have a glass specialist from its structures division examine 22 damaged front and side windshields that were removed from the affected planes, said NTSB air-safety investigator Jennifer Kaiser.
DIA was experiencing highly variable weather at the time of the incidents, with high winds, rapid temperature changes and some snow, Kaiser said.
NTSB is looking at whether a pressure change may have contributed, she said.
“It’s baffling,” he said.
One plane from Great Lakes Airlines encountered the problem as well, Kaiser said.
Yet that would likely have damaged other parts of a plane’s exterior, and SkyWest spokeswoman Marissa Snow said no additional damage was found on her company’s aircraft.
The NTSB’s Kaiser confirmed that there were no reports of impact from flying debris.
Frontier reported that cracks were noticed by employees on two of the company’s planes while they were parked at gates and hooked up to an airport power source, leading to speculation that a power surge might have led to the unusual cracking of the windshields, which have electronic heating elements.
However, the SkyWest planes that suffered windshield cracks were never hooked up to a DIA power source, and the planes were in the process of taking off or landing when the problem developed, Snow said.
He added: “We will find an answer.”
“Thirteen Plus One” from Air Safety Week, February 26, 2007:
For a while, it was sending shivers of triskaidekaphobia (a morbid fear of the number 13) up spines at Denver’s International Airport (DIA). One after the other, 13 windscreens had cracked on a variety of airplanes.
Then much later, safety officials found a 14th plane and the abnormal fear of the number 13 was suddenly no longer a player. It was now a mystery worthy of the X-Files’ Scully and Mulder. At least 55 flights were canceled and others were diverted.
The U.S. National Transportation Safety Board (NTSB) has called in a glass specialist, because the problem also lies beyond their ken. He will examine 22 damaged front and side cockpit windshields that were removed from the affected airplanes. “We are not discounting anything,” says FAA spokesman Allen Kenitzer, “We believe it is very remarkable to have this number of events in the same area at the same time.”
During a recent broadcast on Denver’s 9NEWS, Frontier spokesman Joe Hodas said there may have been a power surge while airplanes were connected to auxiliary power lines at the gates.
Hodas said the surge could have done something to weaken the electronically heated windshields. Hodas says it’s unlikely, but it’s the only theory as of last week. DIA spokesman Steve Snyder says engineers found no electrical problems, and one airline that reported cracks never hooked up to the power grid.
Wind gusts of up to 50 miles per hour were recorded at the airport and there were rapid temperature changes and some snow on the afternoon of Feb. 16, when airlines started reporting the cracked windshields. There was no hail on Feb 16-17 in Denver, just light flurrying.
SkyWest Airlines, a regional carrier for United Airlines, reported nine planes with windshield cracks (Embraer EMB120 Brasilia and Bombardier Canadair Regional Jets), while Frontier Airlines said four of its Airbus aircraft suffered similar damage. One plane from Great Lakes Airlines suffered a cracked windshield, It was confirmed that there were no reports of impact from flying debris, ice sliding from terminal roofs, etc. On one aircraft, a CRJ 700, all four cockpit windows were cracked.
The answer most probably lies in the systems that generate windscreen heating for demist, deice and birdstrike protection. Having windscreens heated for take-off greatly increases the panel’s pliability and reduces the chance of a birdstrike doing much more than spider-webbing the outer layer. So how are windscreens heated?
Windscreen de-icing is provided by “Nesa” glass windscreen panels, consisting of a vinyl core sandwiched between two thicknesses of semi-tempered glass.
These panels vary. CRJ windshields are three panes of “glass” (i.e., layers). MD-11s have seven-layer panes. Some inside layers are acrylics and not “glass”, per se. On the outside surface of the vinyl, between the vinyl and the outside layer of glass is a conductive Nesa coating which provides approximately five to six watts per sq. inch power input.
The windscreen de-icing is entirely automatic and the temperature is feedback thermostat controlled to provide the quantity of heat required for anti-icing, and at the same time, keeping the vinyl layer at a temperature which gives it the best resistance to bird impact. The three forward panes of the aircraft are designed in this manner.
The vinyl centre layer has the additional advantage, that in the event of a windscreen being shattered, the vinyl will still withstand at least twice the maximum differential pressure in the fuselage by blowing out in the form of a bubble.
Aircraft that are cold-soaked (as at Denver) and which only have a single position windshield heat, would probably benefit from a half-hour exposure to APU heated cabin air before windscreen heat was applied. Why so? Well, windshields are held in position by many tight-fitting bolts. It’s not a great idea to start straight-off heating the windscreens when the surrounding frames and bolts are still icy cold and contracted.
Because only the outermost layers were cracking at Denver, it’s a sure sign that the problem was all about the sudden temperature differentials between the inside and the frozen outside, plus the bolts and frames not being given time to release their frozen grip.
However, flight crews wouldn’t want to open their side windows in that sort of weather to spot or communicate with ground-crew, so they’d most likely light off the APU and hit the windscreen heat switch simultaneously, to clear the condensation layer off the inside panels (and the ice off the outside).
Vastly different coefficients of expansion between the metal and the transparencies, starting from sub-zero temps, will do it every time. Theories about heated de-icer fluids being sprayed on cold windshields were also discounted.
This stepped heating arrangement is really as old as jet airplanes. On the Vickers Viscount, the aircraft A.C. supply (invertor system) was derived from two invertors each with a capacity of three K.V.A., with their output regulated to 115 volts 400 C.P.S. These invertors were known as the Normal and Auxiliary invertors. The Normal invertor supplied all the services including the wind-screen Nesa glass, when this was selected to the low position.
Selection of Nesa glass to the high position automatically started up the Auxiliary invertor. On the B757 and B767, the FWD windshield heat is initially (automatically) applied at a low power setting before the power is increased. This is to prevent cracking by avoiding thermal stresses between layers. On the B737-200 series, the control and application of heat is also automatic with a “ramp” voltage being applied, low at first and gradually increasing but automatically, with no manual input.
Lockheed C130, Electra, Orion, and other windscreens had a two position NESA system, plus a low temperature start button. The button-press was designed to minimize thermal shock in conditions of very low outside air temperatures, by applying cyclically phased heating currents until a point near cabin ambient was reached (at which point continuous NESA LOW currents would be applied).
NESA High currents were for inflight icing that the LOW setting couldn’t cope with — and, once airborne, High was also a good idea for low-level flight when the bird-threat was high. Modern designs of one-shot blast NESA can cause windscreen micro-stresses that eventually generate inflight failures under the added cyclic stresses of regular pressurization cycles.
In cold-soaked airplanes, they can go straight to failure mode on the ground, particularly if the feedback thermostat isn’t up to reacting appropriately at sub-zero temperatures — i.e., no feedback means that the system is seeing insufficient heating — so more current is called for. The resultant rapidly stepped up heating would quickly shatter a windscreen.
In fact, that’s the probable scenario for at least some of the failures: a thermostat design that doesn’t feedback at very low cold-soaked starting temps.
It’s a fair bet that an airworthiness directive will eventually emerge, calling for a period of APU inspired cabin pre-heating before windscreen heat is applied, once the outside air temperature is (say) 20 degrees F below freezing (or colder).
One pilot’s advice seems pertinent: “Leave the freaking windshield heat on all the time, even overnight… trying to save a few pennies of electricity on the overnight ends up costing you tens of thousands of dollars for windshield replacement.”
He has a point. Besides the cracked windscreen replacement, lost utilization, customer disenchantment, and general enervation, airline schedules tend to be shattered by such events. It can really be a…pane.
The verdict is in. It was FOD, or “foreign object debris,” that fractured at least 21 front and side windshields on 14 planes at Denver International Airport on Feb. 16, according to air safety investigators.
The pilots of one plane reported taxiing through “some dirt and debris” before the cracking occurred, said Denver-based National Transportation Safety Board investigator Jennifer Kaiser, who led the probe of the bizarre DIA incident.
Wind gusts at the airport reached 48 mph, said Kyle Fredin of the National Weather Service. While winds of that intensity are at the “high end,” they are not that unusual.
Cracks showed up on 14 planes – from SkyWest Airlines, Frontier Airlines and Great Lakes Airlines – within about three hours in the early afternoon.
“The only commonality across aircraft type, operator, location, time and phase of flight was the wind and weather,” Kaiser said.
She said windshield fractures developed on six planes as they were taking off, some of which aborted; on one just after it had landed; on two as they were taxiing to the terminal after landing; on three as they were parked at the gate; on one as it was being pushed back from the gate; and on one while it was at 19,000 feet.
It is possible that the high winds blew back sand that may have been put down on DIA’s runways and taxiways during the December and January storms.
The airport’s snow and ice plan says “sand used in the air operations area will be washed, sharp sand, and screened” to a fine-grained product.
Investigators were not able to determine the precise nature of the debris because there were no “transfer” marks of the material onto the windshields, Kaiser said. “We have nothing at the impact sites to say this is definitively what it is.”