The video above may look like rough water and it sort of is. It is choppy, but small choppy waves may not be the end of the world. The super short distance between the waves reduces the hazard as they are so tightly spaced together they have little pitching effect on the seaplane (keep in mind this is a 9,000lbs seaplane). Extend those same size waves just a little further from each other and you can have very big problems, extend them even further from each other (hundreds of feet farther) and again the hazard is reduced.
Not all waves are created equal
If you’ve ever surfed, you know all about wave period. This next section is for those who haven’t been enlightened into that subject.
We call the wave “period” the time between crests as waves pass a stationary point. This depends on two factors, wavelength (the physical distance between the crests of two waves), and the speed at which they are moving. The period is the time in seconds that wave crests are spread out. In seaplanes (hull or float) we are really concerned with wavelength most of all (but some people incorrectly refer to this as period so we thought we’d tell you the difference between the two).
When being forced to land in open, unprotected water, swells travel up to thousands of miles without anything to stop them and this can result in some very very large waves. You can, however, land a small float plane successfully in an emergency in the open ocean. Let’s say in one scenario I say you can land a J-3 Cub on floats in 20′ waves, and in another scenario, I tell you that 15′ waves are likely to capsize or flip the airplane and may be fatal. Here is the difference….
Height to length ratio
Looking at the top example in the blue diagram above, let’s say the waves are 15′ tall on the top with the crests spaced closely together (tight wavelength), maybe the crests are 105′ apart. This would yield a height to length ratio of 1:7. In the bottom example, the waves are 20′ tall, yet the crests are spaced 500′ apart. This yields a height to length ratio of 1:25. With the greater distance between the crests, we are far more likely to be able to land on the top of one crest and ease down its backside settling onto displacement prior to the next wave. With tightly spaced wave crests (short wavelength or sometimes called a short period), you are much more likely to skip off the top of one wave crest and plow your floats or hull directly into the face of the next wave, resulting in very bad things happening (flipping, structural damage, etc).
Now here’s where “period” comes into play. Surfers measure it as time between waves from a stationary point. That makes sense since they operate from the surface of the water at relatively a stationary point (or a certain distance from shore). For us, we can manipulate the “period” and thus the “wavelength” by choosing to land downswell or upswell. If you land in the same direction the swell is traveling you can effectively give yourself more time before meeting the next wave. Let’s use simple math for an example:
- Wavelength is 500′ between crests
- The swell is moving at 30ish knots, so we’ll call it 50′ per second
- Let’s say it takes you 500′ and about 5 seconds to get your plane from touching to displacement
If you land upswell, the waves are coming at you at 50′ per second. In the five seconds it takes you to stop, you’ve effectively traveled 750′ and went for a rough ride as you landed on one crest and then got picked up (and maybe flipped) by another.
If you land downswell, the waves are traveling in the same direction as you. You land on the top of one crest and have 500′ between you and the next, but in the 5 seconds it takes you to stop, the crest in front of you that you were chasing down and may have impacted has moved 250′ further from you, and you come to displacement prior to getting rocked by the next wave.
It is preferred to land parallel to the swell direction if winds permit.
Landing perpendicular to the swell
Downswell is better.
How to survive
The name of the game is to get the airplane down at the slowest possible speed and not allow it to be tossed back into the air by another wave crest. If you become airborne after touching down you will likely be too slow to fly and the airplane will impact the water and likely flip. It is highly recommended to get very quality instruction from a very experienced instructor who is familiar with open water operations if you plan to ever successfully execute one. Open water landings should be prepared for as with any other emergency, floatation gear ready, doors unlatched, etc.