Gate to Gate

Over the last couple years, I have written about many bits of technology that aircraft use. I really haven't discussed too much what bits are used when, and for what. This post, I will try and tie all the items and their use into a comprehensive post. I'll use a commercial airliner (Part 121) for the discussion, both because they typically use more technology, and because that is what my background is. I will also base most of this on flights in the US, to keep it simpler.

We can start a few hours before the flight actually leaves. As the flight approaches it's scheduled time to leave, a group of folks have started planning for the flight. Sure, there is network planning, they set up the schedules and try to be sure the flight will make money, and provide continuity, and such, but that is usually done months in advance. There are also the simulators that the pilots use for proficiency checks, and training, but that is on going and not related to a particular flight, but important none the less.

Dispatchers and meteorologists are considering the situation between the origin and the destination of the flight. The meteorologists are generalists, looking at the weather over the country, where dispatchers are more concerned with the weather along the route between the cities. The dispatcher needs to consider the situation at the specific airports, for runway closures, and other challenges unrelated to weather. There is a tool called Collaborative Decision Making (CDM), where the dispatchers work with the other airlines so everyone can utilize the airports and airspace as optimally as possible.

The dispatchers can use CDM to look for areas to avoid when selecting a route between the two cities. If there is a bad thunderstorm along the optimum route and all the other airlines are avoiding it to the south, the dispatcher may pick a northern route to stay out of everyone elses way. Once the dispatcher selects a route, they need to build the rest of the flight plan. The dispatcher will build a flight plan using many tools. The dispatcher may allow the flight planning engine to select routes, or the amount of fuel. Depending on aircraft maintenance situation, and MEL deferrals and such the flight planning engine can accurately predict the fuel burn based on weather and route.

Once the dispatchers are happy with route and fuel selected, they will file a flight plan. The flight plan will be filed with the Air Navigation Service Provider (ANSP) for both the origin and the destination. For the US the ANSP is the FAA, in Canada is is NavCanada, and in the UK it is NATS. The ANSP handles all the RADAR and air traffic control (ATC) functions. The flight plan will give the ATC controllers a heads up on what the aircraft was planning on doing once in the air.

As the pilots get to the aircraft, one will typically do a walk around of the outside of the aircraft making sure the aircraft looks safe and no damage is visible. The other pilot will typically go to the cockpit and begin setting things up. There may be a the initialization of the FMS, maybe a RAIM check of the GPS, and entering the flight plan prepared by the dispatcher into the FMS. The dispatcher provided flight plan will usually include weather information for the route, and any other non weather realted information for the route (IE ATC changes, etc), The flight plan will also contain fuel and time information that the pilot can double check, insuring the dispatcher hasn't made any mistakes.

When the pilot know the fuel situation, they may confer with the fueler to adjust any fuel amounts to be put on board. The pilot will also need to know how many bags and passengers are on the flight, so they may make proper weight and balance calculations. Some airlines have a load planner who takes care of the weight and balance, others still let the pilot take care of this. Depending on the aircraft, it may be necessary to have a person dedicated to making the load calculations.

As all the passengers are seated, and the pilot is about to move the aircraft, they will ask for permission to move. There may be a ground controller dedicated to the gate area, and there will need to be taxi clearances and such needed from them. Other airports everything is controlled from the tower, and any movement must be cleared through the tower controllers. An ACARs message may be sent requesting the Pre Departure Clearance (PDC), that will be a version of the flight plan sent to the ATC with any ATCneeded changes to the plan. The PDC will also contain the code the pilot needs to enter into the transponder. The pilot must acknowledge receipt of the message.

After the aircraft is taxied to the runway, the pilot will ask the tower for the final airport clearance, by announcing "ready for takeoff". Once the ATC controller gives the pilot final instructions the pilot can access the runway and start the takeoff. The ATC instructions will be the route the pilot should take to get from the runway to the beginning of the flight plan route. Every bit of the instructions and plans for the takeoff are there in case there is a failure. If there is a radio failure either from ATC or the Aircraft, the instructions given are good enough for the pilot to take off, fly the planned route, and approach the destination. It is a safety situation, should the plan be the safest and most expeditious way to fly the route.

Once the aircraft is above about 300 ft, depending on the  airport, the aircraft will appear on RADAR. The first RADAR that will show the aircraft is the TRACON, who will control the aircraft after tower hands off the aircraft. The TRACON controller will control the aircraft until it is more than 30-50 miles from the airport. The TRACON will hand the aircraft off the enroute controllers who will control the flight until it is 30-60 miles from the destination airport. The RADAR data will be collected and sent to the FAA command center for others to view, and use the ADSI information. As we move into NextGen, there may be more ADS/B position reporting, instead of RADAR.

Once the aircraft is on the route, the pilots will typically engage the autopilot. The autopilot will help maintain the route of flight, altitude and throttle settings to insure the aircraft flies the route planned, and uses the fuel planned. The pilot must monitor the autopilot to be sure it is engaged, and doing the right thing the whole flight. Occasionally pilots will hand fly the aircraft, for practice. Once in a while the autopilot will fail, and the pilots must had fly the aircraft. The systems in the aircraft are designed for certain reliability levels.

During the enroute portion of the flight, there may be messages the pilots need to send to the company operations center. The pilots will usually send a text message over ACARS if they don't have a lot of urgency to the message. The pilots also have an option for voice communication using company assigned frequencies. If there were to be a medical emergency, the voice communications will be used, if a pilot is looking for a weather report for 400 miles ahead, they will use ACARS.

As the aircraft gets closer to the destination, ATC will typically begin having the aircraft start to descend. Newer approaches follow a continuous descent profile, where the pilots set the throttles to idle at altitude, and basically use the potential energy to glide the aircraft to the runway, reducing noise, fuel burn and pollution.

Current approaches typically are designed for the aircraft to provide it's own guidance. That is there are airs on the ground (or satellite) to provide the aircraft the information it needs to know where it is, and fly to the runway. Features like DME and ILS radions are on the ground, and GPS satellites are in the air.

The enroute controller will hand the flight off to the TRACON controller about 50 miles from the airport, where the aircraft will be below about 10,000ft. The TRACON controller will clear the flight for the approach that it will use to get to the airport. At about 5 miles out, the pilot will be told to contact the tower, and the tower and the pilot will make the final checks and be cleared for the runway to land on. Once on the ground, the pilot will talk to the ground controllers to get to the proper parking area, and maybe a gate controller for certain airports. Once the wheels are chocked, and the engines shutdown, the pilots are mostly done with the flight.

Yes, there is a bit of technology going on between each gate, and a little before. Ever think about that before.

Collaborative Descision Making or Why Is My Flight Late

The FAA and the airlines (and some business aviation) have an agreement. If there is no space for the airplane, leave it on the ground. The FAA has a set of systems monitoring all flight plans, scheduled flights and the weather, mushes it all together to determine if there is room for the aircraft in the National Airspace System. If there is too much traffic at a certain point, then the FAA will issue a program, and tell the airlines to keep their aircraft on the ground.

If there is fog at an airport, and the rate the airport can take aircraft in is reduced to less than the volume of scheduled flights into the airport, then the FAA will issue a ground delay program (GDP). This ground delay will cause flights to be delayed at the departure airport. Typically the delay will  begin after the weather starts slowing things down, and will continue until the forecast shows things will get better.

Most passengers find this frustrating, but the reason is quite sound. The flight is going to be late to that airport anyway. Rather than risk stranding passengers at another destination, because the aircraft had to hold waiting for a slot at the destination, and the aircraft ran low on fuel, the passengers are left at the departure airport, and can choose a different itinerary, or just wait it out.

Other programs include an airspace flow program (AFP). An AFP is used when there is a line of weather across several states, causing many flights to be planned to go around that weather, all at a single choke point. The choke point is like an airport, where not all flights can get through, and may have to hold to allow proper spacing. Rather than getting too many aircraft through a small hole, it is better to leave the aircraft at the departure airport, with a delay, rather than possibly holding, and diverting.

Any of the delay programs are just a delay. The flight is delayed a known amount based on forecast weather. Sometimes, the delays can be adjusted based on current local forecasts. Sometimes the weather is better than forecast, other times not.

The last major program is a full Ground Stop (GS) program. This is where the conditions at the destination airport are not conducive to landing any more aircraft. One scenario for a ground stop would be malfunctioning equipment at a destination, and the time to repair is not determined.  The delayed aircraft may get an updated expected departure time, or they will get a next update time.

With the DOT 3hour rules and other reasons, some airlines may elect to cancel a flight or two due to one of these programs. Cancelling flights allow space to be made available. The FAA allows airlines to trade these spaces, or hold them for other flights. If airline A decides that it would be better to delay flight 123 that only is 60% full, they may decide to put most of those passengers on flight 456 that is 50% full, but leaving an hour later. That way the airline can make 48 people only a little late, and they will tell their friends how the airline "saved" them. The other 32 will just be frustrated, and may not complain too loudly.

The swaps and adjustments are part of the Collaborative Decision Making (CDM) program the FAA organizes. The CDM organization actually has several arms, and is chaired both by the airlines and the FAA. Some of the arms include groups working on weather, ground movement, and flight planning.

One of the programs the CDM group is working on, is collaborative flight planning. If the FAA and the airlines could make plans based on know congestion areas, then some of the programs might be eliminated. If the FAA gets a bunch of flight plans that all show the flights going around the south end of a storm, the FAA can suggest that going the north way might get the flight there sooner, even though the mileage is longer.

There is still a long way to go with all the information the airlines and the FAA  have to get passengers to their destination when they want to be there. Things are getting better all the time.

How Can Flight Planning be Accurate?

In pilot training, we all learned to use an E6-B or something similar, along with winds aloft forecast to do our flight planning. The values were pretty good, about as good as we could get considering it was all we knew. If the trip was short (about 500 miles or less) being careful, it can be nailed pretty well.

Now imagine flying a jet, over 2500 miles non stop, flying airways, in or near the jetstream, with an E6-B and the winds aloft forecast. It can be very accurate, but will be very time consuming to calculate. The interpolation alone will be a tremendous effort, and then calculating along the route, and keeping it all added correctly can be quite a challenge.

The computer has a small advantage in calculating the flight plan. Computers are great at keeping track of all the numbers, and interpolating. The National Weather Service (US NWS) and UK Meterological Office (UKMO) all model the weather in little rectangular 3D shapes, that the computer can look at and determine winds and temperature for the airspace.

There are multiple wind models used in various systems for flight planning. The Global Forecast System (GFS) model is pretty good, and available almost world wide. The GFS model is run 4 times a day, and forecasts can go as long as 16 days, with the first day having a specific forecast every hour, starting the second day every third hour up to 8 days, and every twelfth hour up to 16 days. The 3D rectangles are about 27km square and about 500 feet thick. The US also produces a finer grained model, called RAP or Rapid Refresh model which is updated more often has smaller 3D rectangles (13km squares, by about 600feet) and only covers the CONUS for 12 hours. (A high resolution rapid refresh model (HRRR) is being developed with 3km squares).

If a flight plan is created that will take an aircraft from Los Angeles to New York, flying at FL370 using the route:

  OSHNN4 DAG J100 LAS J146 DVC PUB SLN SPI J80 CREEP APE CTW KODIE PSB LVZ LENDY6

The computer will look at all the 3D rectangles that the aircraft will pass through, and using the winds and temperature for that airspace, calculate the ground speed, distance and time for the aircraft in that area. The accumulation of all the times will be accumulated, and the end result will be the total flight time. That result is most of the time, then the book values for the aircraft can be used for climb to, and descend from altitude.

Normally the flight planning computer will expand out the OSHNN4 departure to all the waypoints, and calculate them individually to top of climb (TOC) then start calculating in the in the 3D rectangles at altitude. The flight planning computer will expand out the LENDY6 arrival and determine the top of descent (TOD) calculating everything up to that point using the flight level 3D rectangles, and the proper rectangles on the way down.

For airlines, a dispatcher will normally generate these flight plans. The dispatcher will look at the flights coming up for their area, and maybe run a preliminary plan (just an FYI, the Jeppesen Jetplan Flight Planning Engine takes about 6 seconds to run a plan similar to the Los Angeles to New York above). The dispatcher can look at the flight plan result, and plot it on a map to see where there may be weather or other congestion, and adjust it accordingly.

Once the dispatcher is happy with the plan, they will file it with the FAA ATC. The FAA ATC can look at that plan, and suggest changes or let the pilot fly it as it is. The FAA will keep the plan on file and use it in various calculations, including URET and other collaborative decision making (CDM) systems.

Overall, the computer makes flight planning much easier. There is much more to flight planning, this only covers winds and temperatures aloft, and how it applies to flight planning.

What should I cover next?