What the flow! Why is my flight delayed?

If you’re a frequent traveler to Canada’s busiest airports, you’re likely familiar with a few dreaded terms: flow, ground delay program (GDP), and the worst of all, ground stop.

Photo by Anete Lūsiņa on Unsplash

So, what do these terms mean? While these delays can be frustrating, they exist to keep Canada’s air traffic organized and, most importantly, safe.

Flow

Let’s start with flow. As the name suggests, flow is a method air traffic control (ATC) uses to manage the timing of aircraft arrivals in busy airspace. Airspace and air traffic controllers can only handle so many aircraft at a time, and these limits can vary depending on factors such as staffing levels, weather, and runway availability. To prevent too many planes from arriving simultaneously, ATC institutes flow control.

Without flow, aircraft would have to hold (circle in the air) until the airspace or airport could accommodate them. Holding isn’t efficient and can be frustrating for both passengers and crew. Worse, if an aircraft runs low on fuel while holding, it may need to divert to an alternate airport, resulting in even longer delays.

When a flow program is introduced, pilots must estimate when their aircraft will be ready for departure. ATC then assigns a departure time that avoids excessive holding at the destination. Depending on the situation, flow delays might be as short as 10 or 20 minutes or as long as several hours.

Some of Canada’s busiest airports regularly experience flow programs, particularly during peak times. On clear weather days, these programs typically cause only minor delays, but during adverse conditions, the impact can be more significant.

Ground Delay Program (GDP)

A ground delay program (GDP) serves a similar purpose to flow by managing the number of aircraft arriving at airports operating at reduced capacity. These reductions are often caused by weather—thunderstorms in the summer and snow or ice storms in the winter can drastically reduce how many aircraft an airport can handle.

For example, if an airport usually accommodates 50 arrivals per hour but can only handle 25 due to weather, ATC implements a GDP. Airlines then collaborate with ATC to negotiate arrival slots.

If an airline typically schedules 25 arrivals in an hour but is now allocated only 10, difficult decisions must be made. Airlines might delay flights until a slot becomes available or cancel flights altogether. Factors such as passenger connections, crew scheduling, and aircraft positioning are all taken into account when making these decisions.

Once the airline finalizes its plan, ATC assigns a specific departure time to each flight. However, GDP delays are often longer than flow delays, and conditions can change unexpectedly, requiring further adjustments. If a flight misses its assigned departure time, it could face a long wait for a new one.

Ground Stop

When airport capacity becomes too restricted or weather conditions are particularly severe, ATC may initiate a ground stop. A ground stop means that no aircraft are allowed to depart for the affected airport.

Ground stops can be geographically specific or apply only to certain types of aircraft, depending on the situation. Typically, a ground stop is announced for a specific duration—say, 30 minutes. After that period, the stop may be extended or lifted, depending on the circumstances.

Ground stops often precede the implementation of a GDP, especially when conditions remain challenging.

Final Thoughts

While terms like flow, GDP, and ground stop may cause frustration for travelers, they play a vital role in ensuring safety and maintaining organization within Canada’s busy airspace. Understanding these systems can provide some perspective the next time your flight is delayed.