Twin Otter floats

System Message · Post by **System Message** » Wed Jul 20, 2011 10:42 am

Posters in the damaged Maldavian Twin Otter subject suggested wildley different handling qualities of different float models. Could someone give some more detail here.

Rowdy · Post by **Rowdy** » Wed Jul 20, 2011 5:33 pm

I'm waiting for PanEuropean to pipe up.. but heres my little bit of a take on it.

The whiplines and the CAP floats certified for the twin otter are of two very different designs. The aircraft also sit differently on them. Different speeds and different attitudes for takeoff and landing because of this. I'm sure you've noticed the Cap equipped aircraft all sit in what appears to be a nose low stance and the whiplines sit nose high. The Caps were only certified on stub nosed machines whereas the whips for both the short and long nose. There will always be debate as to which are the better float, but they both have their strengths and weaknesses. I personally have only ever flown the Twin on Caps, so I cant comment.

sean.j · Post by **sean.j** » Thu Jul 21, 2011 1:44 am

Rowdy, How did you find landing on caps, flap-less. Was it challenging was it a non-issue?

Driving Rain · Post by **Driving Rain** » Thu Jul 21, 2011 6:07 am

Flapless landings are prohibited as a normal operational practice. There are a variety of reasons for this, high touchdown speed and inability to successfully carry out a baulked landing in accordance with the certification criteria when operating in that configuration are two of them. de Havilland did not "forget to approve the test", De Havilland did not approve 0° flap landings because they do not provide the margin of safety required by CAR 3 and SFAR 23, the two certification criteria that are applicable to the Series 300 Twin Otter. That's for both sets of floats CAP and Whips.... however the AFM states

Supplement 23 to the Twin Otter Supplementary Operating Data (PSM 1-63-1) provides some guidance to operators when a Twin Otter must be landed with flaps retracted. The title of the supplement includes the very clear notation EMERGENCY OPERATIONS ONLY.

And yes the CAP floats can be hair-raising in a flapless landing due to the way they are mounted with no angle of incedence as compared to the Whips. The sweet spot on CAPs is very small in the zero flap configuration. I recall many pilots at the OMNR sweating this on their PPC's .

Yankee · Post by **Yankee** » Thu Jul 21, 2011 4:20 pm

The Whips have a much wider pitch range and the float mount geometry offers a more advantageous angle of attack whilst on the step. They also have a steeper slope on the afterbody of the float that negates the CAPS afterbody porpoise tendency. Rough water ops are better due to the slower unstick and landing speeds. Some argue that the deeper V float construction is better for rough water, but it would take a hydrological engineer to confirm that one????

The CAPS mounting geometry offers a lower angle of attack when planing on the step. The result is a higher takeoff speed. Not good for rough water ops and flight crew members have (allegedly) used 25-30 degrees of flaps to get the aircraft out of the water sooner. (A no no as per the AFM). In addition the float design offers a very narrow pitch range for the least drag pitch attitude. If the pitch is too high, the afterbody will dig in and spit you out repeatedly as an afterbody porpoise ensues. This is more prevalent in smooth water (high drag) situations.

The CAP equipped aircraft is much nicer to step taxi. Whether it is less buoyant or the wings are producing less lift, I’m not sure, but the CAP Otter feels much more solid when on the step. The CAP Otter takes more finesse, and perhaps that’s why some are die hard CAP fans???

The Whip equipped aircraft is akin to flying a super cub on floats. Almost any pitch attitude will work, albeit ugly.

During the transition from CAPS to Whips at MAT, we routinely swapped aircraft midday between CAP and Whip equipped machines. This led to some entertaining delights of floatmanship at the Male seaplane base. The two aircraft porpoise for different reasons (typically), and fix is opposite. Whip aircraft usually bow porpoise due to too low a pitch attitude on takeoff. CAPS usually afterbody porpoise for too high a pitch attitude. Mix the fix and your finesse goes out the window.

Whip drivers new to MAT would takeoff or land the CAP machines too high and end up skipping in smooth water conditions. CAP drivers would crap themselves while trying to land the Whip with the low CAP pitch attitude. Can you say tuck! And bow steer? I witnessed more than one example of a twin otter trying to repeat the beaver mother lode scene.

Both floats are fun to fly, but as previously mentioned by other posters, you’d be much better off in the Whip if you had a hydraulic failure. I like them both, as they allow my favorite airplane the ability to float, but the CAP machine looks sexier. Isn’t looking good the most important thing anyway?

BTW……MAT did not buy 20 + sets of Whiplines over a short period of time because they looked nice or wanted to spend $5 million bucks. You can operate in rougher conditions, with less (obvious) aircraft damage, with less skilled flight crew members with the Whip.

PanEuropean · Post by **PanEuropean** » Fri Jul 22, 2011 4:52 pm

Rowdy wrote:I'm waiting for PanEuropean to pipe up...

Hi Rowdy:

I can't participate in this discussion or in the other discussion (TMA accident) because it would be inappropriate of me to do so. Plus, I have very minimal experience on floats - I have flown both the CAP and the Wipline floats, but not for sufficient time to enable me to make subjective comparisons between the two with any level of confidence. I spent most of my career flying war relief on wheels (standard and intermediate flotation gear).

Michael

r22captain · Post by **r22captain** » Sat Jul 23, 2011 6:44 am

Pan Euro.
I regret to inform you that we, the members here at AvCanada are formally filing a complaint against you recent actions in this thread. Your lack of wild speculation, rumours, and absence of any information to start a mud slinging, thread derailment has disappointed us all. Are we suppose to continue in an educational direction? WTF?

We don't care about your honesty regarding your lack of time on floats. Please come up with some ridiculous "there I was" stories, and report back.

Thank you.

PanEuropean · Post by **PanEuropean** » Sat Jul 23, 2011 9:43 am

... laughing out loud... That was great, R22.

Michael

Dhc6to8 · Post by **Dhc6to8** » Sat Jul 23, 2011 11:01 am

That is really funny R22... even I had a good laugh!

Seems Michael is a truely honest chap... that is hard to come by these days!