Holding Pattern Computer

Response from developer

The current version of the Holding Pattern APP for both ICAO and FAA methods of flying the holding pattern is identical when using the Along-Track-Distance (ATD) rather than time. The Holding Pattern APP solves for the exact RNAV Turn Point, which is a function of TAS, turn rate, wind speed, wind direction, and ATD. DME Turn Points are replaced by RNAV Turn Points. This allows all aircraft flying different indicated airspeeds and altitudes in the holding pattern to intercept the holding radial at the same ATD. The difference between ICAO and FAA holding patterns relate to the time issue. FAA specifies required inbound time, while ICAO specifies required outbound time. The only limit on the FAA method in order to intercept the holding radial at the correct inbound time while using a constant bank angle is the ratio of the windspeed to the TAS (i.e., the windspeed ratio) normal to the holding radial must be less than 1. In the ICAO method of specifying the outbound time, there is a significantly lower limit on the windspeed ratio normal to the holding radial. This limit is a function of the turn rate and the required outbound time. For example, in the case of a standard rate turn and a one-minute outbound leg, the maximum windspeed ratio normal to the holding radial is 1/3. Any value greater than 1/3 will not allow the aircraft to re-intercept the holding radial with a constant bank angle. Most avionics boxes use a variable bank angle to try to intercept the holding radial. This will work when the wind is coming from the non-holding side. In this case, shallowing the bank angle will allow the aircraft to re-intercept the holding radial. However, if the wind is coming from the holding side and the aircraft is bank limited, the aircraft will always overshoot the holding radial. Thus, it is impossible to intercept the holding radial under these conditions. Again, this does not occur in the FAA method since there is always a unique solution to the RNAV Turn Point Problem. We call this solution HPS (Holding Pattern Solution). The first paper on HPS was published in 2018. The Holding Pattern APP uses the HPS analysis. There are other benefits of using the FAA method for flying the holding pattern, one of which is called High Precision Entry (HPE). In HPS, the aircraft is assumed to enter the hold along the holding radial. In HPE, an additional parameter is incorporated in the analysis, which is the heading of the aircraft tracking inbound to the holding fix. Here the solution of the RNAV Turn Point allows the aircraft to smoothly blend into the HPS ground track, and thus allows the aircraft to re-intercept the holding radial satisfying either the required inbound time or ATD. This capability will be incorporated in the next update of the Holding Pattern APP. The methodology used in the Holding Pattern APP can significantly reduce the holding pattern protected airspace, as well as reducing Pilot workload in the cockpit for both entering and flying the hold. A dialog has been opened with the FAA to convince ICAO of the benefits of switching over to the HPS/HPE methodology. If ICAO converts to the HPS/HPE method of flying the holding pattern, they can still use their current maximum IAS holding speeds versus altitude tables, since HPS/HPE only requires the TAS, turn rate, windspeed, wind direction, and either inbound time or ATD.