Demystifying Weather Delays

For most people, weather is just a happy circumstance or a frustrating nuisance. For pilots, reading weather patterns is part of the job. They must decipher myriad sources of forecasted weather information during the flight planning process to determine routes, altitudes, fuel consumption, and time to destination.

When Mother Nature disrupts the flight plan, a highly skilled pilot will make adjustments to determine the best course of action. Training providers like FlightSafety International (FSI) ensure pilots understand how to weigh factors like time, distance, fuel, passenger convenience, and safety to create the right plan for any weather condition.

ADJUSTING FOR DENSITY ALTITUDE

Density altitude can effectively shorten runways—but pilots can train to accommodate this factor. Essentially, density altitude indicates the altitude that the aircraft “thinks” it’s flying at based on atmospheric conditions. As an aircraft climbs into higher, less dense air, its performance generally decreases due to many factors. At high-density altitudes, aircraft need more runway to take off, cannot climb as quickly or as steeply, and need a longer landing roll.

The three factors that affect density altitude are the elevation of the airport, outside air temperature, and humidity, with humidity having the smallest effect on aircraft performance. A high-elevation airport such as Aspen (7,820 feet) on an 82˚F summer afternoon may have a density altitude of more than 10,500 feet, even with a relatively low 17% humidity. This means an aircraft taking off or landing at this airport would perform as if it’s at more than 10,000 feet—and operations can become more challenging when the temperature rises.

“During simulator training, the pilots perform density altitude calculations for operations in these [high and hot] conditions and then we have them actually fly these scenarios,” said Richard Meikle,  Executive Vice President Safety, Courseware, and Compliance at FSI. “If you’re used to fairly rapid climb-outs away from airports, and then the instructor positions the simulator to a high-density altitude airport, the performance is noticeably degraded. The instructors can adjust the simulated weight of the aircraft and the temperature to allow clients to experience and understand the different performance levels.”

While density altitude affects all aircraft to some degree, it affects piston-powered aircraft more than turboprops or jets simply because of the latter’s higher thrust-to-weight ratio.

“Not all airplanes are created equal,” said Meikle. “Sometimes it’s very difficult for an owner or passenger to see some aircraft departing from an airport and hear from your pilot that your aircraft can’t depart because it doesn’t meet the performance requirements.” 

ADJUSTING FOR COLD OR PRECIPITATION

In the opposite extreme, cold weather can also affect an aircraft’s performance. Frost on the wings can alter the airfoil shape and disrupt airflow enough to considerably hinder the generation of lift. This is why it’s important to hangar your aircraft and/or ensure it is completely de-iced prior to takeoff in frost-forming conditions. Anti-icing systems also draw power from the engine’s bleed air system, reducing engine performance. This can especially be a factor at high-elevation airports.

In addition, wet or icy runways can decrease airport capacity or even prevent specific aircraft from landing at all. During preflight planning, pilots note the length and condition of the runway(s) at the destination airport to ensure that the aircraft they are flying can stop in the given distance. They will take many factors into consideration: whether the runway is grooved (providing additional traction and water runoff capability) or ungrooved, wind direction, airport altitude, density altitude, and the required length of runway for the given conditions.

However, even with this information, runway excursions account for more than a third of all aviation accidents each year.

“Landing on a runway that has now become wet when the airplane performance doesn’t support it is extremely dangerous,” said Mark Kleinhans, director of safety for FSI. “The two best ways to handle this are to either divert to an airport that has a longer runway or wait for the weather to pass and the runway to dry. Either option may inconvenience the passengers but is better than a damaged aircraft or worse.”

ADJUSTING FOR WINDS AT TAKEOFF AND LANDING

Since airflow over the wings (or rotor blades in the case of helicopters) generates the lift that enables the aircraft to fly, wind is an important element during takeoff and landing. A headwind straight down the runway is preferable as it assists in the generation of lift on takeoff and helps slow the aircraft on landing. A tailwind down the runway is undesirable since it negates lift on takeoff and pushes the aircraft on landing (causing a longer, more squirrelly landing roll). In addition, crosswinds can push the aircraft off the runway if they are strong enough.

“High, gusty winds low to the ground can cause turbulence and wind shear, which make it very difficult for pilots to keep a smooth, stabilized approach to landing,” said Meikle. “If an airplane is drifting, floating, too fast or too slow, too high or too low on final, the pilots often elect to go around and set up for another approach. Pilots are trained to execute go-arounds, which are much safer than trying to force an aircraft down in an unstable condition.”

Sometimes the impetus of a go-around starts well before the final approach. Air traffic control demands that seem benign 100 miles out can result in an unstabilized approach. For example, if air traffic control (ATC) keeps your aircraft at a higher altitude than planned to allow another aircraft to pass under you, by the time they let your aircraft descend, your approach is steeper and faster than planned. With some high-performance aircraft, that speed is difficult to bleed off in time to make a good approach.

Meikle says that pilots are mission-oriented, and they want to complete the planned flight as much as the passengers want to be on the ground. “It’s important that pilots are recognized for making the good decision when they go around, regardless of the cause,” Meikle said. “If they’re in a position where they feel like the go-around was the best option, they should be thanked for making the right decision.” 

ADJUSTING FOR THE NATIONAL AIRSPACE SYSTEM

Even when the weather is perfect in the area you’re leaving, you may be hit with delays due to weather in other parts of the country.

“Storm fronts can extend hundreds of miles across from multiple states and affect an entire region of the country,” said Meikle. “The most extreme thunderstorms can tower more than 55,000 feet, which is higher than the service ceiling of any passenger or business jet.”

Since these thunderstorms (technically called cumulonimbus clouds) generate updrafts and downdrafts of tremendous force, and hail that can cause significant damage to an aircraft, ATC often route traffic—both commercial and private—hundreds of miles out of the way when a storm front of sufficient height and ferocity cuts across a planned flight path. The rerouting can affect hundreds of flights, creating pinch points at the areas north and south of a storm front, which typically travels from the west to the east in North America (hurricanes and other localized wind patterns can disrupt this usual flow). The rerouting increases ATC’s workload as it provides multiple course and altitude changes to hundreds of aircraft to work them safely through the pinch points.

Weather directly affecting major airports also plays a role in the national airspace system. As the weather deteriorates and visibility decreases, the number of aircraft that a particular airport can handle also decreases. For example, at Chicago’s O’Hare International Airport—one of the busiest in the world—an aircraft typically lands about every 30 seconds for a visual capacity of about 119 arrivals per hour. But when visibility is less than 3 statute miles, the airport capacity can drop to as low as 94 arrivals per hour. This means that upon arrival at the airport, some aircraft must circle and wait for their turn to land.

“In this situation, instead of stacking aircraft in long orbits at different altitudes near the airport, ATC may hold aircraft that haven’t departed yet on the ground,” said Meikle. “They can either call a ground stop—which prevents any aircraft on a flight plan to land at specific airports from departing anywhere in the U.S.—or hold all aircraft in a specific region.”

While ground holds may be frustrating to passengers, they are safer than in-air orbits and may ultimately help the passengers reach their destination. Consider an airport like Teterboro, NJ, a popular destination for business aircraft. In very close proximity, there are three major airports with high volumes of airline traffic. When weather approaches the area, the normal departure and arrival paths may be cut off and limited routing options exist for safe operations. This is when ATC will implement a ground delay and hold programs to ensure there is sufficient space for aircraft to safely navigate through the gaps in the weather.

“Aircraft on ground holds use little to no fuel, depending on how long the hold is and when the pilots are notified,” said Meikle. “Whereas aircraft can be in holding patterns for so long that they reach a critical fuel level and must divert to a different airport to land safely.”

Passengers can check the status of the national airspace system and delays at specific airports at https://nasstatus.faa.gov/.

PREPARING FOR THESE ADJUSTMENTS

One way to reduce anxiety caused by weather delays is to build a buffer in your schedule. Just as pilots plan their flights with at least 45-minute fuel reserves, schedule your flight with a comfortable “time reserve” to account for any unexpected delay. You can also discuss weather and diversion options with your pilot before takeoff. Diversion decisions often need to be made well in advance of landing to arrange for ground transportation, since you’re not landing at the designated airport.

“It’s much easier to have a discussion with your pilot on the ground if there’s any possibility that the flight may have to divert, especially if your destination is in a mountainous area,” said Meikle. “A more suitable airport 10 miles away in a straight line may look great to your pilot, but may add more than an hour of drive time for you if you’re driving on mountain roads, and you might have a better alternative that works for both of you.”