A flight plan route represents a 4D profile comprising the following elements:
- lateral position in latitude and longitude (e.g. waypoint, navaid)
- vertical position (altitude)
- time since takeoff
In order to ensure the orderly flow of traffic, ATC has to know about the full 4D profile to expect. This chiefly depends on your aircraft and its performance. Historically, flight plans only contain the ICAO identifier of the aircraft which ATC can use to estimate a 4D profile. Obviously there is a big difference between a C172 and a A320. In addition to the aircraft type, the cruise speed is also part of the route and can be adjusted throughout the route.
What is missing completely is climb and descent performance. This is mostly relevant to evaluate Eurocontrol restrictions. If there is a bit of airspace ahead that you are not allowed to enter and at a certain distance before the airspace, you start to climb, then it is important to understand whether your aircraft will have the required climb performance. Climbing 1000ft in 10NM at FL100 is no problem for a B737 but a real challenge for a C172.
Additionally the taxi time is relevant as a flight plan only contains off block time and in order to allocate airspace correctly, it is important to know if the aircraft can depart within 5 or 30 minutes after off-block.
Eurocontrol employs an aircraft performance modelling database called BADA (Base of Aircraft Data) which for each ICAO type contains some modelling, giving cruise and climb performance at different altitudes. For some aircraft (mostly commercial air transport), the quality of the model is very good. For others it can be poor, especially since only a few GA aircraft are actually modelled and others are treated as synonyms. The main difficulty comes when one ICAO identifier applies to different aircraft with very different performance. This is mostly found with piston aircraft where a turbo-charged or turbo-normalized variant exists, either from the manufacturer or as an aftermarket modification. Turbocharging a piston airplane completely transforms its performance characteristics at altitude. Until late 2014, the best known cases were the Cirrus SR22 and the Socata TB20/TB21. The ICAO type SR22 was used for both the normally aspirated and the turbocharged SR22 until the new type SR22T was introduced. The type TRIN used to refer to both the TB20 and the much more powerful TB21 for which now two types exist with different performance data.
For a lot of other piston aircraft, the problem still persists. For example the type C82R applies to both the normally aspirated Cessna R182 and the turbo-normalized TR182 and the BADA profile in this case chooses to model something that is closer to the TR182 than the R182. Many GA piston airplanes have aftermarket turbo-chargers (e.g. Rockwell Commander) through STCs and there is obviously no special ICAO type for those. Even the venerable C172 exists with different engines (Reims Rocket, Hawk XP, etc.). Therefore Eurocontrol’s approach to performance modelling via a custom database comes with limitations.
Another factor influencing performance (mostly climb/descent) is aircraft mass. This information is not part of the flight plan and always assuming MTOM can yield inaccuracies.
In addition to BADA, Eurocontrol also maintains a standard taxi time database giving typical taxi times for each aerodrome. In some cases you might want to specify a different taxi time depending on the specifics of your operation but flight plans do not contain that information.
Custom Flight Profiles
As a solution to BADA inaccuracies, Eurocontrol introduced support for custom 4D profiles as a flight plan extension as well as information about takeoff mass and taxi time. The remarks (RMK) of the flight plan can contain the following keywords followed by a colon (:) and a value:
- BOC (Bottom of climb)
- TOC (Top of climb)
- TOD (Top of descent)
- BOD (Bottom of descent)
- DAL (Distance at location)
- TAXI (taxi time in minutes)
- TOW (takeoff weight in kg)
When validating a flight plan in ICAO format, these values override Eurocontrol’s internal model – provided the values are complete and considered to be not too far off from the internal model. A sample flight plan may look as follows:
(FPL-ZZZZZ-IG -1P28R/L -SDFGLORY/S -EDDS0925 -N0138F090 OKIBA3B OKIBA DCT GIBSA DCT AMUGI DCT OBISI P605 DLE DCT DOKOK DCT AGATI P605 NOLGO NOLGO1A -EDDH0306 -DOF/150107 RMK/TOW:748 TOC:D8F050T0005 BOC:D14F050T0009 TOC:D25F090T0017 TOD:D289F090T0235 BOD:D307F040T0244 TOD:D329F040T0259 DAL:343ADEDDH TAXI:7 PBN/B2D2 -E/0508 P/TBN R/E A/WHITE BLUE STRIPES)
For the vertical markers (BOC/TOC/TOD/BOD), a distance from the departure aerodrome in nautical miles, an altitude in flight level and a time from departure in minutes is given. Additionally, a distance at location (DAL) in nautical miles for the destination aerodrome is specified. This is required by Eurocontrol for the profile to be processed.
autorouter and flight path profiles
With the exception of takeoff weight, autorouter always generates a complete flight path profile using the performance data from your aircraft settings. The flight plans are displayed with the elements in the RMK section so that you can cut and paste the plan and use external validation facilities (a flight plan might not validate without the profile data because the Eurocontrol profiler might have a different idea about the trajectory and see the aircraft violate restrictions). The taxi time is taken from a database maintained by Eurocontrol and provided to autorouter. If you want to change the taxi time, you can edit the flight plan directly and modify the value. Takeoff weight is not passed at this point as we have observed that a large percentage of users do not correctly specify the weight & balance of their aircraft or do not specify the actual loading so in order to not cause any harm by incorrect information, we omit this field for now.
When filing a flight plan, the profile information is not displayed to the user as autorouter transmits it to Eurocontrol directly and not as part of the remarks section. It would therefore be regarded as duplicate information and given that it is a relatively new addition, it might be unknown to recipients of the flight plan and cause confusion.
Finally a word of advice: the profile completely depends on your aircraft configuration. Make sure that what is specified in the aircraft settings actually matches real world performance of your aircraft. This not only results in a realistic 4D profile but also a useful navlog as part of the briefing pack.