Water-Asssisted Landing

Water-Assisted Landings

Water-assisted landings may be considered one of the more treacherous off-airport maneuvers that aviators rely on. Essentially, it’s waterskiing a wheel plane to shore to achieve the shortest possible landing. 

What is the difference between waterskiing and a water-assisted landing? Waterskiing is a non-landing, even-keeled operation conducted at a higher speed. A water-assisted landing is a landing technique that involves waterskiing for a short distance with a deliberate speed reduction before reaching the shoreline to minimize rollout. Think of it as extending the approach end of the airstrip out onto the water. The touchdown point is on the water, as close to shore as possible, but far enough out to stabilize the approach and achieve the desired deceleration effect. This requires much greater precision and speed control than waterskiing.

Learning water-assisted landings by trial-and-error is a high-risk approach. A safer option is to combine study, training, and conservative practice to gain proficiency and help the pilot understand and anticipate the range of effects waterskiing can have on a wheel plane. Here I’ll share what I’ve learned and how I calculate a safe minimum waterski speed, factoring in both current speed and direction. 

Warning: this technique is only recommended for tailwheel aircraft conducting wheel-landing water touchdowns.

What are the hazards?

  • Wheel penetration and nose-over; sinking
  • Cold water immersion/drowning
  • Steeply sloped shorelines, cutbanks 
  • Submerged objects (large rocks, logs, boulders, structures, and machinery), powerlines, tram cables, snags, small icebergs, headwind gusts, birds
  • Landing in a place that is too short to take off safely

Water-assisted landings are more hazardous than waterskiing due to slower groundspeeds, on-the-water speed reductions, and intentional water penetration used as a deceleration technique. In contrast, waterskiing an airplane involves higher consistent power settings and ground speeds which minimize water penetration, rolling resistance, and the forward pitching moment when water contact is made. 

What is the Purpose?

  • Enable the shortest possible landing on beaches, outwashes, and gravel bars
  • Add another skill that could be helpful in the event of a forced or precautionary landing
  • Increase the margin of safety. Even if the landing site can be landed normally, a water-assisted landing will shorten the rollout and help prevent a possible overrun. 
  • Stabilize the approach before reaching the shoreline and prevent a long touchdown
  • Develop broad-spectrum proficiency and expand emergency skills

Water-Assisted Landing Techniques

Prepare for a water touchdown by first running a couple of simple calculations. A few new terms are introduced here.

Calculate a safe minimum groundspeed

Use this formula to calculate a safe minimum ground speed before touching wheels to water:

Vwater  = Vp + Vc + Vm

Where:
Vwater   = waterski speed. Minimum water touchdown ground speed.
Vp   = penetration speed. The airplane’s speed, relative to water, at which point the airplane’s main wheels begin hopping or progressively penetrating.
Vc   = current speed. The speed of the current component that is aligned with the landing track. River current speed is typically estimated and varies widely in each stretch of river. The effects of a crosscurrent can also be calculated (see below). Landing upstream yields a negative value.
Vm  = safety margin. Additional speed that is added for safety (a subjective value).

Example 1: Upstream Landing 

Landing Direction: Upstream
Current Speed: 6mph (consistent speed)
Flow Direction: Parallel to landing track
Penetration Speed: 45 mph
Safety Margin: +5 mph 

Vwater = 45mph – 6mph + 5mph
Vwater = 44mph

Example 2: Downstream Landing

Landing Direction: Downstream
Other conditions same as above.

Vwater = 45mph + 6mph + 5mph
Vwater = 56 mph

Example 3: Crosscurrent Landing 

(see crosscurrent components formula below)
Landing Direction: Upstream Crosscurrent
Current Speed: 6mph (consistent speed)
Flow Direction: 60 degrees to the landing track
Penetration Speed: 45 mph
Safety Margin: +5 mph 

Vwater = 45mph – 4mph + 5mph
Vwater = 46mph

More about penetration speed (Vp)

Waterski Penetration

A key safety concern is knowing Vp for your airplane and preventing sudden penetration. Venturing close to this speed is flirting with disaster. 

A higher rate of descent and a slower ground speed will increase the forward-pitching moment and drag when first touching down on water. If touchdown occurs at or below penetration speed in deep water, a crash is likely. The only possible escape is immediate full throttle w/aft stick. 

How is penetration speed determined?

This is the six-figure* question. Get out there and practice in still, shallow water in calm wind conditions. Water should be deep enough to sense the transition from waterskiing to hopping or penetration, but not so deep that it could cause a crash. Start in a safe, shallow area and transition gradually to deeper water until the results are conclusive. Operating in shallow lake waters close to shore could be an option. A video camera can be used to record flight information for later reference. 

Factors that influence water adhesion, rolling resistance, and penetration speed include:

  • tire size and inflation pressure
  • water surface texture
  • current speed
  • crosscurrent component
  • rate of descent on touchdown
  • gear loading
  • groundspeed

Factors that reduce penetration speed:

  • larger tires 
  • lower tire psi 
  • lower gross weight
  • low gear loading (elevator-induced)

How to prevent sudden penetration?

While Vp is calculable, pilots must learn to anticipate and sense impending penetration and be ready to make early corrections to prevent it. This typically involves adding power to increase speed and airflow for greater elevator authority and making elevator control inputs (aft stick/yoke) to unload the main gear and raise the nose.

More about current speed (Vc)

How to evaluate a crosscurrent

A crosscurrent exists when the current direction is not parallel with the airstrip. In other words, crosscurrent has a perpendicular component to the water landing track. Think of it like a crosswind. The difference is we’re looking at water current and calculating the headcurrent component. This information is a factor in (Vwater) calculations. 

Anytime the current angle is greater than 25 degrees to the landing track, crosscurrent effects should be considered. Here’s how to calculate it.

First, calculate the current speed. This can be done using a float to measure the time it takes to cover at least a 20’ distance. Use the formula speed = distance/time to calculate current speed, or use a flow meter. 

Now measure current angle relative to the landing track. This can be done with a compass.

Finally, calculate the headcurrent component. I like to use the iPhone scientific calculator. It’s fast and easy: Headcurrent component = current speed x cos(current angle) or, using the example below, 10 x cos(55) = 5.7 mph (since the landing direction is upstream, Vc is negative). You can also use an E6B app or crosswind calculator. I sometimes use an app called xWind for this purpose. And it’s handy for visualizing the compass rose and finding reciprocals.

Here’s an example:

Landing Direction: Upstream
Current Speed: 10 mph (consistent speed)
Current Angle: 55 degree crosscurrent 
So, the Headcurrent Component = -6 mph.

Assess the Landing Site

Look for gradual, waterline transitions that are safe for water-assisted landings. Avoid abrupt edges, steep slopes, and large rocks. Be sure the bar or beach is clear of debris and obstacles. 

Landing upstream is a safer option if conditions permit, particularly on shorter bars, so that in the event of an overrun into the water, the current won’t sweep the aircraft away from land. Note any current speed or direction changes in the touchdown and rollout zone that could change your calculations.  

Prepare for a Water Touchdown

You’ve calculated (Vwater). Now it’s time to kick the tires and light the fire. But wait, there are a few more things to consider.

Plan the Rollout

After touching down on the water, additional power may be required to overcome rolling resistance and maintain the desired ground speed. Focus should be outside, not on the instruments. Be ready to add power and come back on the stick/yoke. Forward pitching, deceleration, or a hopping sensation all demand more power

Pull back the power just before reaching the shoreline. Reducing the power farther offshore to intentionally penetrate the water can provide additional deceleration and will shorten the landing roll, but use caution in deep water.

There’s no need to stretch it out. The safest approach is to minimize the distance the airplane is waterskied before reaching the shoreline, while still benefitting from the effects of deceleration.

Water Braking 

When tires meet water, they spin up and may spray water on the underside of the wings or only the empennage. Brakes can be applied to mitigate this, but they should be released before reaching the shoreline. The hydrodynamic effects and potential benefits (Vp reduction) of water braking are an open question. I don’t bother water braking. I find it distracts from more critical operations. 

Other variables worthy of consideration include: 

  • water surface texture
  • gust factors
  • down drafts/cold air sinks
  • wind shadows
  • shallow water effects

The effects of shallow water, however inconsequential, could be further explored. In general, an increase in drag occurs when transitioning from deep to shallow water for two reasons. 

  • The Bernoulli Principle – water passing below the tire will speed up more than in deep water as it is displaced horizontally. This creates an area of reduced pressure resulting in more sinkage.
  • Wave-making resistance – the tires generate a more significant bow wave

Final Thoughts

Current speed and your airplane’s penetration speed are two critical pieces of information that need to be right. Get either of these wrong and you could end up dead in the water. Endeavor to make careful field observations and calculations lest you sink the ship and drown. We don’t rely on guesswork when operating in an airport environment. Why should we in the field?  

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*Typical value of a well-equipped Super Cub.