One of the most ingenious of the devices intended for use in night landing, especially emergency landing, is a very simple optical instrument known as the Jenkins night altitude indicator. The design and operation of this instrument, which allows a pilot to determine the altitude of the aircraft, is discussed. The author discusses various modifications and improvements that might be made to the instrument

Warner, John A C

English

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TECHNICAL NOTES 
ADVIRORY OO~On'T.'TFE FOR AERONATJrTCS. 
No, 123. 
AN OPTIC~i.L ALTITUDE: INDICJ.TOR FOR NIGHT LANDING. 
By Joh~ A. C. Wa~ner, 
B-~:r.ea.l.:'.. of Sta.ndC'yrds. 
January, 1923. 
https://ntrs.nasa.gov/search.jsp?R=19930080908 2020-03-17T04:16:11+00:00Z
NATIONAL AbVISORY COr.~nTTEE FOR AERO_TAUTI CS. 
TECHNICAL NOTE NO. 123. 
AN OPTICAL ALTITUDE I NDICATOR FOR NIGHT LANDING. 
By John f . C. 1~ar~er, 
Bureau of Standards. 
That practical co~~ercial aviation has come to stay 
must be admitted by even the most Iskeptical. The rarid ad-
v3.nce ,,.hich the past few yeevrs hav~ witnessed is unmistak-
~bly the forerunner of greater a8tivity in this cornparative-
ly new field of communication and transportation. It is at 
once evident, h07ever, that the greatest benefit cannot be 
derived from the use of aircraft as commercial carriers 
unless their operation can be extended over the full twenty-
four hour day; for their inactivity during the hours of 
darkness robs them to a great extent of their advantage 
over the systems of ground trans~ortation. 
For this reason the pToble~ of flying at night and 
under other condi tions of 10'''' visibility is now demanding 
the attention of many aeronautical experts. We must equip 
our airways and aircraft with suitable means for surmount-
ing the obstacles offered by these adverse conditions. 
This of course involves the installation of markers and 
beacons to clearly define the routes and fields, and also 
the equipping of aircr8,ft with sui table instruments for 
navigation and landing. One of the most ingenious of the 
devices intended for use in night landing, es pecially emer-
~__________________________ _ _ ~ ______________________________________ --1 
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gency l:md.ing, is a very simple optical instrument known as 
the Jenkins night altitude indicator. 
Referring to Fig. 1, we note three projectors: A, B, 
and C, ea ch of which is equipped wt th an incandes cent lamp 
properly mounted at the upper extremity to project a beam of 
light downward through the tube to the ground or water upon 
which a landing is to be made. T~o of these projectors: B 
and C, are a t tached rigidly and. ::>aralleJ. to each other to 
the side of the aircra:t, wh~le the third, A, is made rotat-
able (upon rails D and E) through a certain angle in a plane 
parallel to the fore-and-aft axis of the ship. ]~otion of 
A is brought about by the aviator who manipulates a hand-
'!vheel oper3.ting through a pinion mating with rack F. 
Projectors Band C are each equipped with an object 
screen 1,vhich provides a characteristi c image on the landing 
surface; as shown in Figs. 2 and 3) a rectangular bar is 
projected by B and two blunt arrowheads by C. The ground 
im~ge from A is the altitude figure representing the par-
ticular ~ltitude for which the projector is set . 
In determining the 3.1titude of the aircraft the pilot 
simply turns the wheel attached to the pinion mating with 
rack F until the light-beam from A intersects that from 
C at the landing surface. As A rotates, a toothed metal 
disc G 3.ttached to A and extending through the walls of 
the projector tube into the light-beam is caused to rotate 
in a definite manner by virtue of the action between the 
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tee"[;~ of G and those of the fixed ra.ck ~vi th which they en-
gage. Tbe I'o~;atlnG disC" ac-t;a o.,l::1 a J:otc1.!;2.ble ubjec'~ screen, 
for it is pierced by openirgs in t~e fo~m of the altitude 
nU'1le:r.als cor r.eeponn..l.ng to the part iC1.:l1a:c 8e t'Chlg of the pro-
jector. Thus it ie that the in:ag'J Deell "'~pO~1 the lo..ncllng 
su:r.face between the a.rrowheaclEl p1'0jected by C in that of 
the al ti tude nume:z:a1s cu'~ in G, thh)ugh wl1~ oh the 1 "l.ght 
passes. 
The a1 ti tude ma.y also be observed on the trcmsp:l,1'ent 
scale H, for an opening in the case cuntaining .. .;he ilhln1-
inating element of A allow3 a beam of lig}1"~ to fall upon 
the scale grac.uation whj.ch corresponds to the paJ.'ticule,I' 
setting of the pr-oJeotoI' at which the ground itnE1.f,(;s are 
seen to meet. The intersecting beams (from A and C) furm 
two sides of a triangle wilose al ti tude, determined ',y:l'i;h the 
instrument, is also that of the aircraft above the landing 
surface. 
Inasmuch as 50 feet is the lowest direct indication 
of a1 ti tude for the instrur.lent des cribed, the illuminated 
bar image projected by the fixed source B is employed in 
estimating aJ.titudes of less than 50 feet. It will be 
seen that as the aircraft approaches the ground with a.ll 
three projectors stat:'onary, the m .. ll'ne.ral "SOIT will move 
from the arrowheads toward the bar image. The prevailing 
altitude is then estimated by observing the positIon of the 
altitude image with respect to the bar and arrowheads. 
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For example, an aJ.titude of 25 f~et wouJd be indicat8d when 
the numerals wore observed. rl1j.d.'!'9'a:r tet ;veE:n the other two 
images as shown by Fig. 2. The m£:',iI.i"'lum dil'cct l no.ioa-'cion 
of the inst ~~u:ment is 500 f;:; 8t (;J 88 !"lg. 3) . 
The night al ti tude i nd:.uat,o:-: de:·H.1ribed has 1een ueed 
in Great Britain. Tests of the i.n8t~ument co:n:iucted in 
this country have sho~n satisfa~tory result.s. * 
The Germans have developed several types very similar 
in principle to the Jenkins device. One of the most int-
eresting of these involves the projec'liion of a beam down-
ward and fo~wa~d from a light-source fixed to the tail of 
the aircraft. Diffusely reflec~ed r~ys are. in turn thrown 
upwa.rd from the landing surfa.ce and pass through an optical 
arrangement in th~ cockpit where the pilot may observe his 
altitude by noting the position of a, spot of light against 
a transparent scale, 
Various possible rnodlfiC3.tions and improvements of the 
above instru~ents are readily apparent, For example, the 
Jenkins indicator might be simplified by omitting the second 
projector (B) whose advantages are of doubtful importance; 
for at altitudes below 50 feet a pilot would gener~lly pre-
fer to watch the l~nding area ahead of him rather than to 
observe the ground images and estimate their relative 
positions. 
Further simplification might be effected by having both 
projectors (A and C) fixed to the aircraft in definite posi-
*Bureau of Standards te3ts con~ucted in flight and in the lab-
oratory by A. H, Mears and J. B. Peterson. 
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tions. In thIs case the beams 0: light would intel'sect at the 
13.nding surf~ce for only the one chosen altitude to which the 
arrangement had be~n ~djusted. Other altitudes might be esti-
m~ted by noti.ng the separation of the ireages. For such pur-
~oses. it would be desi~able to have the projected images 
characteristic of their source; other vise difficulty would 
arise in re~dily determining whether the fo~· 'TTard or aft 
im3,ge vvas leading. 
To the -:;ri ter 1 s knowlecige no tests have been conducted 
in this country to deter:'!J.ine the charactel'is ti cs of the 
Germ3.n adaptations as mentioned e_bove. Ho "levex J one might 
reasonably doubt the feasibility under all conditions of 
using the diffusely reile cted l'ays from a landing surface for 
other than direct observations. 

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Indica tion at 25 feet 
Jenkins ni gl~t 3.1 ti tude indica-t;o~ 
Fig. 2 
------------ ~ -------- - - - -
Indication at 50 feet 
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Indication at 500 feet 
Ground images 
of 
Jenkins night altitude indicator. 
Fig. 3. 
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An Optical Altitude Indicator for Night Landing

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