8.7 Descent, Vortex Ring State and Landing

The Condition Schedule 4 Names by Name

Vortex ring state (VRS) — also called settling with power — is the one multirotor aerodynamic hazard the Part 101 MOS singles out for both avoidance and recovery. It is priority A, and it is examined.

What happens

In normal flight the rotor draws air from above and accelerates it downward, and that column of downwash falls away behind the aircraft. In a vertical descent, the aircraft moves down into the very column it has just created. The rotor now ingests air that is already moving downward.

Two things follow. First, the induced flow through the disc rises sharply, which reduces the effective angle of attack at each blade section and cuts thrust — the same mechanism as recirculation, but self-generated. Second, the airflow around the disc organises itself into a doughnut-shaped vortex ring: air flows down through the centre of the disc, out at the edges, up around the outside, and back over the top into the centre. The rotor is now churning its own wake in a closed loop and is producing very little useful thrust at all.

The three conditions

VRS requires all three of these simultaneously:

1. A high rate of vertical descent — the aircraft is descending into its own downwash faster than the downwash is escaping.
2. Little or no horizontal airspeed — a descending aircraft moving forward flies out of its own wake continuously and cannot establish the ring.
3. Power applied — the rotors must be actively producing downwash for there to be a wake to descend into. An unpowered descent cannot enter VRS.

Remove any one condition and VRS cannot form. That is the whole of both the avoidance technique and the recovery.

What the pilot sees

• An increasing rate of descent that does not respond to power.
• Buffeting and vibration as the disc works in turbulent, unsteady flow.
• Loss of control effectiveness — roll and pitch inputs feel mushy because the disc is not producing clean thrust.
• On telemetry, high current draw with a continuing descent — the motors are working hard and achieving nothing.

Why Adding Power Makes It Worse

This is the central counter-intuitive point, and it is the one the exam tests.

The instinctive response to an unwanted descent is to add power. In VRS, adding power increases the downwash the rotor is producing — which strengthens the very column the aircraft is descending into, intensifies the vortex ring, and increases the descent rate. Pilots describe the aircraft as "falling faster the harder you pull".

The same logic explains why VRS is most dangerous close to the ground: the pilot pulls power, the aircraft accelerates downward, and there is no altitude left in which to try anything else. Most multirotor VRS accidents are landing accidents.

Recovery: Go Sideways, Not Up

The recovery is to break condition 2 — establish horizontal airspeed so the rotors fly into clean, undisturbed air.

1. Apply forward (or lateral) cyclic — move horizontally. Any direction will do; forward is usually easiest to judge. The aim is to fly the aircraft out of the descending column of air it created.
2. Do not add power while still in the ring. Power added before horizontal flight is established makes the condition worse. Once the aircraft is clearly translating and the descent has arrested, restore power normally.
3. Accept altitude loss. Recovery costs height. If VRS is entered close to the ground there may not be enough height to recover, which is why avoidance matters far more than recovery technique.
4. Once clear, re-establish a normal descent profile — slower, and with a forward component.

Avoidance: The Descent Profile That Cannot Enter VRS

Prevention is entirely a matter of descent technique:

• Descend along a slanted path, not vertically. A descent with a forward component continuously moves the rotors into fresh air, and the wake is left behind.
• Keep the descent rate moderate. Manufacturers commonly advise limiting vertical descent rates to a few metres per second; check your aircraft's flight manual for its figure.
• Land into wind. The headwind gives the aircraft airspeed even when its groundspeed is zero, so the rotors are always meeting undisturbed air. This is the single most effective habit.
• Never descend vertically at speed over a landing point. The common bad habit — climb to height, transit, then drop straight down onto the pad — is exactly the VRS recipe.
• Be alert on the lee side of obstacles, where descending air from a rotor or downdraft adds to the aircraft's own descent.

Landing into wind: the other advantages

Schedule 4 topic 10(c) asks specifically for the advantages of landing or recovering into wind, and there are four:

1. Translational lift is retained. With a headwind the rotors meet moving air even at zero groundspeed, so the rotor is more efficient and less likely to enter VRS.
2. Lower groundspeed on arrival. The wind provides part of the deceleration, so the touchdown is gentler and the aircraft is easier to place accurately.
3. Better directional control. A headwind gives a fixed-wing weathercock stability and gives a multirotor a consistent, predictable tilt to hold rather than a shifting one.
4. A shorter and steeper approach path over the ground, which matters when there are obstacles on the approach.

Pre-Landing Checks

Schedule 4 topic 10(d) asks for pre-landing checks. A workable set:

Finish the approach with the aircraft in sight and the pilot's attention on it. The last two metres are where most landing damage occurs, precisely because the flight feels finished before it is.