Grob Twin Astir

The Nutmeg Soaring Association of Freehold, NY has 3 Twin 1's,
and has sold another one, for a total of 4 which we have owned.
We retrofitted one with a disc brake IAW TM 315-50 last spring.
That one still has the old cast aluminum parts in the gear cage.
After a few months of operation with the new disc brake, a side
load
managed to bend the disc plate, and render it inoperable. Our
mechanics deemed the part to be either substandard or under-
designed and filed a complaint with TOST through Wings and
Wheels. It took a few months, but TOST responded by replacing
the bent disc with a new stronger one, and also said they would
replace other older defective discs in the field. We have
corresponded with at least one other Twin 1 club operator in
Germany who told us that they previously had a disc brake, had
problems with it, and went back to the drum brake to avoid the
previously noted problems. So, it seems entirely possible that the
Utah late serial number Twin 1 was originally produced with a
disc
brake, but later changed to the drum brake configuration due to
an
under-designed disc brake plate. If you get a new disc brake
from
TOST, it should now come standard with the thicker reinforced
plate. Ours has been fine since we installed the beefed up plate.

Mike Opitz

On thinking this issue through a little further, we have come to the
conclusion that it would be pretty easy to tell if the Twin had a
previous disc brake installation. The disc brake uses a hydraulic
reservoir located underneath the rear seat pan. If there was a
previous disc brake in the glider, the reservoir, or at least evidence
of the FRP mounts will be there to see. If one finds this evidence,
then a disc brake installation would be a restoration to the OEM
condition, and not need a 337 form. Also, if the logbook history is
complete, there would be an annotation of a change from the
original disc to a drum brake configuration. Hope this helps..

Mike Opitz