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National Advisory Committee for Aeronautics, Report - General Specifications Covering Requirements of Aeronautic Instruments

All indicating instruments refiuired in the navigation of aircraft
should be as com act, rugged; an '0 t as is consistent with accuracy,
reliability and. urability, and wi ease of reading. Such instru-
ments must be free from the influence of the following disturbing
eiiects, excepting, of course, those effects on which they depend for
their operation, viz, vibration, change of altitude andchange of tem-
pereture.

Barometers or altimeters must be sensitive and of open scale, and
the lag in their operation should be the absolute minimum obtainable.
When operating in a fog it is essential that the distance above the
surface should be known Within very Close limits. Such instru-
ments, of course, are dependent on barometric pressure and on varia-
tions of barometric pressure from the time of the start of a ' ht
until the completion of a fii ht, which can not be provided for, ut
aside from this error their in ications should be substantially accurate
once they are adjusted at the int of de arture. It is, therefore,
necessary that the scale should e of equ divisions, as otherwise a.
change of zero to meet change of barometric height will introduce an
error. Their location on the ai lane must be carefully chosen so
that their indications will not be influenced by the velocity pressures
in flight.
COWASS.

Compasses should have as high a directive force as is consistent
with restricted dimensions. Provision should also be made in the
compass mounting for compensan for the presence of magnetic
material in the constructmn of the airplane, particularly compensa-
tion for heeling and di ing errors._ _ In order that the directive force
shall not be abnorm y reduced by such compensation it is, of
course, desirable that the structure 8 ould avoid the use 0 magnetic
materials in moving parts near the compass location, such as the
control columns, shafts, and leads.