Low-Level III: Wind-Induced Drift

Several things can go wrong when navigating at low altitude. For example, the wind can change, the aircraft may fall behind the timeline, checkpoints may be missed, and damage, weather, or fuel can cause the aircraft to divert.
When issues occur, the crew should be able to compensate for wind, heading and time disruptions to resume safe navigation. This article discusses how wind effects can be quickly approximated, a useful skill when diverting.

Wind

The weather in DCS is fundamentally static but, when dynamic weather is fully implemented, things may change. For now, as long as the flight plan is properly prepared and followed, the wind effect should already be accounted for. However, if the aircraft needs to divert at any point during the flight, some calculations are required.
The Royal Institute of Navigation suggests the following simple but effective technique: the Clock method. To better understand it, let’s first clarify how wind works.
Wind can be seen as a vector with a norm and a direction. Depending on these two parameters, the aeroplane may be pushed sideways or gain or lose speed. In some extreme situations, the aircraft can even remain still in the air, with its ground speed reduced to zero.

Depending on the angle difference between the aeroplane and the wind direction, different effects and means to compensate for them can be observed. The two simplest cases are:

1. Wind in the same direction, or following the reciprocal. In this case, the aircraft is accelerated or slowed down by the full force of the wind. The effect on the aircraft track is minimal. To compensate, the aeroplane can reduce or increase its speed to a value similar to the wind speed.
2. If the directions are orthogonal, ergo the wind comes from the beam, the effect on the speed is minimal. However, the aircraft is pushed sideways and must compensate by turning into the wind whilst travelling forward.

The tricky part is accounting for a combination of wind direction and speed that affects both the aircraft’s speed and its track.

Determining the Maximum Wind Drift

The first step is assessing the magnitude of the wind-induced drift. Starting from the groundspeed value in knots, the crew determines how many nautical miles are covered per minute. For example, at 420 kts, 7 nm are flown each minute, since 420 nm/hour divided by 60 minutes equals 7.
Given a wind, for example, of 330 28 kts, the crew calculates the maximum drift by dividing the wind speed by the number of nm covered each minute. Ergo, 28 / 7 = 4°. Note that, since the speed is the denominator, the faster the fighter flies, the less it is affected by the wind-induced drift.

The Clock Method

The “clock method” allows for approximating how much the drift will affect the aeroplane’s heading. Every 15° between the wind heading and the aircraft heading, a quarter of the drift value is applied. Ergo:

• Relative 15° from the nose → ¼ drift applied;
• Relative 30° from the nose → ½ drift applied;
• Relative 45° from the nose → ¾ drift applied;
• Relative 60°+ from the nose → full drift applied.

Note that the wind direction does not matter as the aircraft always turns into the wind.
Continuing the previous example, if the wind is heading 330 and our aircraft is heading 360, we find that half of the drift value (4°) should be applied. The turn is executed towards the wind, so our aircraft should turn to 358 to compensate for the wind-induced drift.

Wind Effects on Speed

The effect on speed is assessed using the same method but reoriented 90°, thus using the beam as a reference rather than the nose. Moreover, the wind speed is considered rather than the maximum drift:

• Relative 15° from the beam → ¼ speed applied;
• Relative 30° from the beam → ½ speed applied;
• Relative 45° from the beam → ¾ speed applied;
• Relative 60°+ from the beam → full speed applied.

As determined previously, the wind is coming 30° from our nose, ergo 60° from the beam. Therefore, the full speed of the wind is applied to our aircraft. From the true airspeed of 420 kts, we find a ground speed of 420 – 28 = 392 kts. If the correct schedule is to be maintained, the aeroplane should accelerate by 28 kts.

Intuitively, if the wind blows parallel to the aircraft, then the effect on the heading is minimal, and it is maximum on the speed, since the wind is pushing forward or backwards the airplane. If the wind is orthogonal instead, the situation is reversed: the aircraft is pushed towards the left or the right, thus affecting the heading whilst the effect on the speed is minimal.

Practical Examples

Let’s see a few examples. All of them assume zero magnetic variation. To make things more interesting, I compared the values reported by the tool I developed versus the clock method.

Example I

Example II

Example III

I am actually surprised by how close the results are, and how easily the new course and speed to fly are obtained.
When in the cockpit, the calculation of the wind angle becomes even simpler thanks to a tool we should already be familiar with: the BDHI. After serving us so well in endless discussions about intercept geometry, the Bearing, Distance, Heading, Indicator proves itself invaluable once again.
Since the BDHI always shows the aircraft pointing towards the top of the display, figuring out the wind angle and its complementary becomes a matter of seconds.

If the aeroplane is not flying on the planned course yet, the BDHI loses its main advantage, but it is still a valuable reference in case no other tools, such as a spider card, goniometer, or compass, are available.

This concludes this quick look at one technique the crew can use to manage the flight and unplanned events. Following videos will propose methods to manage other resources, such as time.
If there is enough interest, I can collect this and other helpful notions in dedicated kneeboard pages.