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Subject: Re: Instrument repair
From: Ken Gebhart (gebhart@XXX.XXX)
Date: Wed Dec 18 2002 - 22:58:44 EST
To all:
Having just returned from the Paris Boat Show, I see a lot of messages about
aircraft sextants and bubble horizons — something I have been involved with for
the last 43 years. Rather than answering each message, here are some general
comments:
1. Mr. Eno is right about the use of averagers on land - there is no need for
them. They were made to compensate for the natural and regular motion of airplanes
as they wallow through the air (dutch roll), and experience control inputs from
pilots (either auto or human). They were not intended for rough air, and certainly
not the irregular motion of the waves at sea.
2. Most manuals say that the bubble should be twice the size of the sun. This
allows for relative ease in estimating the center of the bubble, while ensuring
adequate mobility of the bubble which is in contact with the upper and usually
lower lenses of the chamber. This is predicated on the fluid in the chamber being
xylene, which it is in all aircraft sextants except the Kollsman periscopic
sextant which has a silicone fluid, as does the C. Plath marine bubble chambers.
We have not heard of a recommended size for these.
3. The accuracy reported by Mr. Kolbe in use of the A-12 sextant is indeed
remarkable. The FAA Flight Navigator Practical Exam allows for an average error
for a series of sights to be no more than 4 miles, with the maximum for a single
sight to be less than 7 miles. The accuracies here are cumulative of (a)
instrument error, (b) user estimation of the bubble center, and (c) user ability
to keep the bubble centered within the horizontal confines of the chamber while
holding the sextant in his hands. If the sextant is propped up, and a star
brought down to the top of the bubble and then to its bottom, and the two readings
averaged, this will remove the two latter errors and reveal the instrument error.
We have found repeatability of these readings to be within one-half mile.
4. The pellicle problem of the Kollsman sextant is a real one and the owner
should bake the pink crystals until blue, and return them to their holder to keep
the pellicle dry. Once too humid, the pellicle will deform and will not recover
no matter what. We have replaced pellicles with thin microscope slides (just
glued them on). This yields two bubble images, so the user would have to
calibrate to one or the other.
5. Mr Dullea should not center the body in the bubble of his AN-5851-1 sextant
since the bubble only touches the top lens of the chamber, and the bubble is
considered “opaque” Its manual says to place the body alongside the bubble.
6. Mr. Hirose may peel the foil covering jacket from a modern C size battery to
make it fit in his A-12. All dry cell batteries in WWII had slide-off cardboard
jackets, which in the case of the A-12 were to be removed. The C sized batteries
with jackets on have always been the same size.
7. Mr. Eno is right again about the RAF MK IX A and B being the best sextants.
We have handled probably every aircraft sextant ever issued operationally, and
they all have some inherent drawback - some design flaw (such as the opaque bubble
in the AN 5851-1), except the MK IX. It is not very elegant looking, but its
simplicity is very attractive. As a result it is remarkable easy to work on.
Anyone can do it, save fixing an averager. The B model is the same as an A model
but with an external averager conected by a shaft. About 5 strokes with a hacksaw
on this shaft will convert a B model into an A. BTW, Celestaire has operational
manuals for sale for almost all aircraft sextants ever made.
Ken Gebhart
Peter Fogg wrote:
> Robert Eno wrote:
>
> > ... The only person I know of who know/knew
> > how to fix these things was a fellow in Australia who may or may not be with
> > us anymore. He was over 80 years of age ten years ago. I have not heard from
> > him in a while.
>
> If that was Max of Edwin Bowers & Sons Pty Ltd he did pass away a few months
> ago. Apart from being a very nice bloke he will be sorely missed as the only
> person known who could repair nautical instruments: sextants, cronometers and
> compasses. The last time I saw him he had a huge ship's compass on his bench he
> was tinkering with. It seems he was the second generation of a family business,
> but his own son died before him so that dynasty, and business, is extinct.
>
> And here I was thinking that the United States may be the closest place to now
> find someone with those skills? If anyone knows of another Australian please
> let me know.
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