This is the first in a three-part post about diversions, while diversions are uncommon, they do happen, in my opinion, on average once out of 3000 flight at my airline. There are generally three main causes for diversions: weather, which I will be the topic of today’s post, mechanical problems, and passenger issues. Each post will start out with a scenario that describes the type of diversion; these scenarios are from my past experiences. I hope you enjoy.
Imagine this. You’re on an airplane and you are approaching your arrival city of Chicago, it’s nighttime, you see that there are some thunderstorms off in the distance. You see the lightning coming out of the clouds and they are getting closer. Just as you notice this, the captain comes on the intercom and says, “Ladies and gentlemen, from the flight deck. I’m sorry to inform you that we are to be entering into holding do to some whether at the airport. There are thunderstorms at the airport and the no one is able to land at this time. We will try our best to sit and wait out the weather but it looks like it might be a while. We will keep you updated from the flight deck until then please remain in your seats because it could get turbulent as we approach the weather. I have also asked the flight attendants to be seated for their safety.”
You are glued to your window and you are watching the lighting outside and feeling the jolts of turbulence as you gradually feel the airplane doing circles in the sky. After about 45 minutes of holding the captain once again comes on the intercom and says; “Well ladies and gentlemen the airport is still not opened and unfortunately we have ran out of holding fuel. We still have plenty of fuel on the airplane but we are going to have to divert to another airport. Instead of landing in Chicago will be diverting to Indianapolis. Our hope is to get some more fuel and get on our way back to Chicago once the weather clears. Again thank you for flying at Acme airlines and we will do our best to get you home tonight!”
If you are a leisure traveler the chances are you’ve never had this happen to you before. If you are a business traveler, or someone who flies quite frequently, chances are you have heard of, or have experienced this exact same scenario. If airlines could control the weather we would; and remember no one ever wants to divert including the airline.
Weather is the number one cause for diversions. Between thunderstorms, winter storms and fog; they all prevent the pilot’s ability to land the aircraft at an airport. Lets break these weather events down further. We will start with thunderstorms.
If there’s a thunderstorm over the airport you can’t land the airplane-it’s just not safe. There have been many crashes throughout the mid-70s and 80s that were attributed to microbursts, which are byproducts of some thunderstorms. Thunderstorms also reduce the visibility and create strong winds and even wind shear. If there is wind shear present while an aircraft if trying to land the flight could loose the airspeed that is keeping the airplane landing. There are instruments on board the aircraft called EGPWS that will give the pilots warning when they might encounter this wind shear. This video explains the EGPWS system.
Winter storms, like a snow or ice storm, have two things that affect the ability of the pilots to land. The first thing is the visibility, which I will discuses with fog. The second obstacle to landing is the runway conditions. As most of us know, snow and ice create slippery roads, which means it also creates slippery runways. As you can imagine it is very difficult to stop an aircraft on a slippery runway. Quite often during a snowstorm the airport authority needs to close the runways so they can clear them and treat them. If there is more than one runway at the airport they will usually alternate them. This means only one or two runways will be open which reduces the amount of airplanes that can takeoff and land. When the airport authority closes a runway it is usually closed for about 15 to 20 minutes while the machines get on and clear the runway. If you have never been on a runway before think if it is clearing a 4-5-lane highway that is 1-2 miles (1600-3200m) long and you have to clear it from edge to edge. If able airport authorities will plan the runway closures for times where the arrivals and departures are low. The video below shows a runway team in action.
Another weather phenomenon that could cause diversions is fog. During heavy thick fog when the visibility is really low airplanes just can’t land. Every instrument approach has a set of landing limits or landing minimums. The crew cannot attempt an approach if the weather is below the published minimums. Due to today’s modern technology, diverting due to fog is becoming less and less common. Most modern airliners have the ability to land with just 600-foot (75M) visibility. In most cases, these CAT III landings are being flown by three autopilots on the aircraft with the pilots just monitoring to make sure the aircraft is doing what the pilots tell it to do (to my pilot readers, I know it’s a lot more difficult than that, and I admire your skill). Some of the regional jet airplanes do not have the ability to fly these lower landing min approaches. If the visibility is too low then these airplanes will not be able to attempt an approach. In many cases when an airport goes below CAT I landing minimums ATC will issue a ground stop for CAT I only aircraft.
Landing Minimums section of an approach plate
The final weather phenomenon that could cause weather diversions is wind. Strong winds coming from specific directions could cause strong crosswinds. All aircraft have crosswind limits, but it also depends on how the airline got certified and how they train their pilots. Most crosswind limitations range between 28 and 40 knots. The rudder effectiveness determines crosswind limitation. If the wind is too strong the rudder will not be able to straighten the aircraft to land and the airplane could land sideways. A side loaded landing could cause damage to the landing gear or even cause the aircraft to crash. In many cases crosswind landings are not a temporary condition. When there’s a strong crosswind the airport could be shut down for hours at a time. To counteract this weather phenomenon, most airports have runways in multiple directions. However there are many major airports around the world that only have parallel runways. When these airports get a strong wind perpendicular to their runway complex, there is no other option but to divert to another airport. I know we have all seen the videos on YouTube of strong crosswind landings but there’s one below in case you have never seen a crosswind landing. The airport is the old Hong Kong airport Kai Tak. This airport closed in the late 90s but a large mountain at the end of the runway created a great vantage point for watching aircraft land.
I hope you enjoyed part one of this three part series about diversions. Tomorrow I will be talking about the next largest contributor of aircraft diversions; enroute mechanical failures.