PIREP

Differentiating FMS Wind Inputs and Blending

Several questions have come in from operators asking about the different methods of wind inputs and if one method is better than another. Typically, inputs can be made by entering a wind value in the PERF INIT page. Winds can also be entered at specific waypoints using PERF PLAN under the NAV key.

In the past, winds would be keyed in manually to coincide with the data provided from the flight planning provider. But with today’s datalink-equipped aircraft, winds are automatically uploaded and inserted into the flight plan as part of the preflight entries. That leads pilots to ask how the FMS prioritizes and blends winds from the different input sources. This article will look at each of the entry methods and explain how the system processes them in order to determine the best methodology.

Entering a Single Cruise Wind – Blending with Sensed Winds

Winds can be entered in several locations and by several methods. The first and most common method is done on the performance initialization (PERF INIT) page of the FMS, usually on page 3 or 4 (depending on FMS). As shown in Figure 1, the page contains an entry for cruise winds and an entry for the altitude. It is important to note that the FMS uses several wind and temperature models to extrapolate for altitude/distance but that is beyond the scope of this discussion.

This was the most common entry method before datalink. The other method involved entering winds manually, one waypoint at a time.

cruise winds entry

Entering winds on the performance initialization page is quick and easy, however winds are blended over the course of the flight plan. The blend is a combination of sensed wind (once airborne) and entered wind. At the present position, sensed wind is weighted at 100%, which means the wind entered gets no priority. The blend projects ahead somewhat linearly: at 200 NM sensed wind is blended 50% with the entered wind. At 400 NM the blend is reversed and sensed wind is only weighted at 20% while the entered wind gets an 80% weighting. Wind blending at other waypoints between these distances is simply provided as a ratio of what is described above. This provides a relatively good average and accurate fuel predictions for most operations.

The second method of wind entry was done on the the PERF PLAN pages after the flight plan had been entered in the FMS. The PERF PLAN pages show fuel and time to each waypoint along the route and are not modifiable from the PERF PAGE display.

perf plan

The Winds and Temperatures (W/T) selection to the right of each waypoint can be selected by pressing the line select key adjacent to the corresponding waypoint, which opens a WIND/TEMP page (Fig 2) and shows several key pieces of information about that fix. Below the waypoint, it will show the altitude the FMS is predicting to be at when it arrives there. To the right, it has a wind and temperature displayed.

wind temp

If a single wind value was entered from PERF INIT, as mentioned above, the blended result for that waypoint will be located here. If not, the entry will read direction and speed of zero and the pilot can enter a wind for this waypoint by typing it in the scratchpad and line selecting it adjacent to the wind value. This will result in the FMS using that wind value from that point in the flight plan until it encounters another entry. This makes it easy to break a long flight into segments by entering a few wind entries every few waypoints in the flightplan. Winds may be entered at each waypoint; this should result in the most accurate time and fuel predictions.

Uplinking Winds

Previously, this method was much more cumbersome, since each entry had to be added manually, it took considerable time to go through the flight plan packet and manually key in the wind entry for each waypoint. Fortunately, the advent of Communication Management Units (CMUs) now allow pilots to retrieve flight plans by recall number and insert the winds via uplink (Fig 3).

datalink option

This has gone a long way toward increasing efficiency and the accuracy of flight planning. Since winds are issued globally every 6 hours, they are generally uploaded once and are set for the duration of the flight. On extremely long flights, it may be necessary to re-upload with updated winds if the duration of the flight crosses into the period.

Hopefully by this point the difference between the two methods of entry is clear. But is one method better than the other, or should both methods be used?

In most cases, if a flight is shorter in duration and there’s no issue of meeting reserves, entering a wind in PERF INIT will provide acceptable accuracy. The worst scenario for accuracy (besides an incorrect entry) is an extremely long distance to the first waypoint, where the system is forced to project only sensed winds due to not having a waypoint to blend. This can occur when cleared to a fix far from the present position or when coasting out.

Like any equation, the answer will typically get more accurate with more data points. So for longer flights, where fuel numbers and reserves are more critical, entering a wind at several waypoints or uploading them is the best option.

The final question that is then asked, is if one way is good and the other is better, is entering both best?

Not really, because when uplinking winds, they are propagated from the first waypoint on. There is really only a slight advantage to doing when the first waypoint is significantly far away. Remember that the FMS is using only sensed wind for the first 100 miles, so only waypoints after that benefit from a blend. If cleared to coast out point soon after departure and that waypoint is more than one hundred miles away, it might be advantageous to use both methods, but typically it is unnecessary.

Please contact Honeywell Flight Technical Services with any questions or operational issues.

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