Smashed windshield -- how pilots deal with cracked glass

Last week it was reported that an A319 just airborne out of Chicago O'Hare had to make an emergency return to land after the flight deck windscreen was smashed by hail. Whilst images in the media portrayed a dramatic event for those on board, how serious really was the event?

Dealing with thunderstorms and severe weather is an integral part of the job of an airline pilot. In addition, dealing with non-normal events such as smashed windshields is also a key part of ensuring that all on board the aircraft are kept safe.

So how does an aircraft windshield shatter like this and how serious an event is it for the pilots?

Windshield structure

Unlike the windscreen on your car, the windshield on an airliner is much more than just a sheet of glass. It needs to protect us from the harsh environment on the outside, be strong enough to withstand the impact of large birds but also give us great visibility to see where we're going.

On the Boeing 787 Dreamliner which I fly, the windshield is made up from three distinct layers. The outer layer is in fact the thinnest layer and is made of strengthened glass, taking advantage of the high damage tolerance of the wonderfully named Herculite® II. This layer of glass sits on top of a much thicker layer of stretched-acrylic.

However, between these two layers is a very clever system.

At cruising level, the temperature outside can be as cold as -90 degrees Fahrenheit. When a windshield at that temperature comes into contact with moisture in the form of rain or clouds, it will immediately get covered in ice. Not ideal when we're coming into land.

The rear windscreen of your car is heated by a grid of wires to get rid of any early morning frost. However, covering the flight deck windows in wires would drastically reduce our visibility.

To overcome this problem, there is a thin layer of a transparent electrically conductive substance between the outer glass and the stretched acrylic. When an electrical current is applied to this layer it heats up, warming the outer glass. With this warmed up, water does not freeze on the windshield. In fact, when sat on the ground in rain, we often see steam coming off the windshield.

To give the windshield added strength, there is another even thicker layer of stretched-acrylic as the base layer.

Aircraft windshields are designed to take a serious beating, be it from birds or hailstones. The three-layer structure as described above means that they can withstand the impact of a large seagull at 400 miles per hour.

Altogether, the windshield is around 1.5 inches thick. Not only does this give it the strength it needs to withstand the worst the external environment can throw at it, but it also makes it light enough to help keep the weight of the aircraft to a minimum, reducing fuel usage and as a result, carbon emissions.

Thunderstorms

The king of weather phenomena, cumulonimbus clouds, referred to as "CBs" in pilot chat, should never be taken lightly. It’s estimated that there are around 40,000 thunderstorms around the world in a given 24-hour period. They are synonymous with heavy rain, hail and severe turbulence.

Thunderstorms need three elements to form. A source of moisture to give them substance, a source of lift to help them grow and unstable air to continue that growth. As a result, when meteorologists see these three factors are present, they can quite easily predict that thunderstorms are likely to form.

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Moisture

Clouds are just condensed water vapor that forms around microscopic nuclei -- anything solid that can act as a core, for example, smoke or dust particles. For any cloud to form, there has to be a source of moisture. This normally comes from large expanses of water such as oceans and lakes.

Lift

Once there is moisture in the air, it needs a source of lift to allow it to climb and form a cloud. This lift comes from variations in the air density, which is affected by a number of elements.

During the day, the sun heats the surface of the earth. As the surface heats up, some of that heat is transferred into the air. However, this heating isn’t uniform as some areas heat up quicker than others creating columns of rising and falling air known as thermals.

In the U.K., there tends not to be the heat from the sun required to form thunderstorms in this way. Instead, they are more likely to occur as part of a frontal system. Fronts are the boundary between warm and cold air masses and therefore areas of different air density. The colder, denser air lifts the warmer air ahead of it. If this air is moist, thunderstorms will form along the front.

Instability

Even if the air has a source of lift, unless it can keep growing, thunderstorms will not form. This is where instability comes in. If an object is pushed upwards and continues to rise, the air is considered to be unstable. It normally occurs when cold, dry air sits on top of warm, moist air.

When these conditions occur, if a mass of the warm, moist air is pushed upwards, it continues to climb. If you remember from earlier, as height increases, the air cools. When this mass of air starts to cool, some of the water vapor condenses to form the distinguishable thunderstorm clouds. As this vapor cools more, it falls as rain and sometimes as hail.

How pilots avoid thunderstorms

As your pilot, our job is to keep you safe and comfortable. As a result, we will do everything we can to avoid the worst of the weather and keep the flight smooth. For us to do that, we have a few tools at our disposal.

Forecasts

Before your pilots even step onto the aircraft, they conduct a thorough briefing that includes the weather en route and at the destination. If thunderstorms are forecast, they will take more fuel. This enables them to deviate off their planned route to avoid storms and hold at the destination until the storms have cleared.

Weather radar

Once airborne, even with the best of forecasts, the system is extremely dynamic and thunderstorms can form in as little as 30 minutes. This is fine if they are well defined during the day, as they are fairly conspicuous. However, at night or when shrouded by other clouds, it’s a different matter.

To help us detect large clouds in advance, we have a weather radar system. The system sends out a high-frequency pulse which bounces back off water droplets. The denser the water droplets, the greater the return on the pulse.

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The radar returns then show the detected cloud on our navigational display. From this picture, we are then able to decide how best to avoid them.

What happened in the United A320 event, I will not speculate on. Like any system, the weather radar can fail. ATC sometimes places restrictions on the route we can take to avoid larger clouds or the weather was worse than the radar returns indicated. There are a whole host of reasons which could be the cause of the incident.

What this event does prove, though, is how strong the aircraft structure is and how well crews are trained to deal with damage to the aircraft.

Managing the situation

Box the chimp

I've had a windshield shatter during a flight and, as you can imagine, it comes as quite a shock. With a sudden rush of adrenaline, the chimp is out of its cage and the instinctive reaction is to jump straight in and try to fix the problem. However, it's no good fixing a problem if you fly the aircraft into a mountain at the same time.

I've also flown through very heavy rainfall and the noise is incredible. It can be so loud that you are unable to even hear your colleague next to you. Add to this a suddenly smashed windscreen and the workload suddenly goes through the roof. As a result, the most important thing to do first of all is to fly the aircraft.

The pilot flying the aircraft will clearly state that they have control and ensure that the aircraft is flying in a manner and direction which keeps it safe. Only once this has been done do we move on to dealing with the problem at hand.

Window damage checklist

Depending on the aircraft type, the manufacturer will provide a checklist in the quick reference handbook (QRH) as to how to deal with a cracked windscreen. On the 787, it's a fairly straightforward procedure that involves removing the electrical power to the window heating system. Once this has been done, we then need to assess the damage to the windshield.

Due to the strength and thickness of the windshield, any damage done by hail is most likely to just be to the outer glass layer. However, we must still ensure that the structure of the windshield is still intact. The best way to do this is to check the pressurization of the aircraft.

If the cabin altitude is slowly increasing, there's a good chance that it is being caused by an air leak from the broken windshield. If this is the case, we continue with step two of the checklist and descend the aircraft down to an altitude where it is safe to breathe without extra oxygen.

Landing with limited visibility

Fly into any major international airport and I’d be confident in saying that you flew an ILS -- instrument landing system -- approach. Developed to give greater accuracy when approaching the runway, the best ILS approaches allow pilots to fly their aircraft all the way to the runway, without even needing to see the ground outside.

The ILS consists of two radio beams which project up from the area around the runway up into the approach path. These signals are then picked up in the aircraft by the ILS receiver which displays them on the screens in the flight deck.

The first signal is the localizer, radiating from antennae which sit at the end of the runway. This shows the pilots where the aircraft is in relation to the centreline. The second signal comes from antennae to the side of the runway, around 1,000 feet in from the threshold abeam the touchdown zone. This is the glideslope and it sends another beam into the sky, normally at an angle of three degrees to guide the aircraft down vertically to the correct touchdown spot.

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Most ILS approaches are flown with the autopilot doing the flying and the pilots monitoring the systems. When the required visual references are seen, the pilot flying will disconnect the autopilot and land the aircraft manually.

However, if we are not able to see the runway clearly, either due to low cloud or indeed a double smashed windshield, we have the option of doing an autoland. When landing in foggy conditions, even when the visibility is as low as 250 feet, we are still able to land the aircraft.

The autopilot remains engaged all the way to the touchdown, including whilst the aircraft is decelerating on the runway. By leaving the autopilot to do the physical flying, both pilots are able to give all our attention to making sure the aircraft is doing exactly what we want it to be doing. If the slightest thing starts to go wrong, we will notice immediately and take the appropriate action.

As the aircraft passes over the threshold of the runway, the autopilot raises the nose slightly to slow down the rate of descent. Quite often, auto-lands are fairly agricultural. The autopilot is great at putting the aircraft in the right spot, it just sometimes lacks some grace. With the wheels firmly on the ground, we can apply the reverse thrust and allow the braking system to slow the aircraft down.

All this time, the autopilot is still keeping the nose tracking down the centreline of the runway. When we have reached taxiing speed and have identified a runway exit, the autopilot is disengaged and the toughest part of the arrival begins -- the taxi to the gate.

Bottom line

While a smashed windscreen may look alarming to those looking in, for the pilots flying the aircraft, it's normally a relatively straightforward issue to deal with. The main challenge will come when landing the aircraft as visibility out of the windshield will be limited.

If it is so bad that we are unable to see the taxiways clearly, stopping on the runway and being towed to the gate by a tug is a valid option. Whatever the decision we make, it will be based on years of hard-earned experience and thorough training all focused on keeping the aircraft, and its occupants, safe.