WW2 Japanese Pilot’s Dead Reckoning Calculator with Neck Cord & US Identification Tag

This was an early Japanese Aviation Tool used by both Army & Navy pilots.  This was state of the art technology then and is a must-have for your Japanese pilot display.

It is boldly marked 時角計算器 on the face. This means “hour angle calculator.”  In the West, this is called a dead reckoning calculator.

This instrument was used to estimate the current position of a Japanese aircraft while in flight.  It does this by calculating the distance traveled from an aircraft carrier or airfield based on the aircraft’s speed, time in flight and direction of travel. It relies on known factors like starting point, course, and speed to determine the approximate position. 

It is made of a large wheel, marked A; a middle wheel, marked B; and a small wheel, marked C. The three wheels are permanently connected. The plastic arms are notched, not broken. They were purposely notched at the factory to align with the readings on wheels A, B & C.

Because this is critical navigation instrument and because pilots needed easy access to them, these were made only 6” in diameter X 1 1/4” thick. At this size, they were easily worn around the neck of pilots.

All the rotating dials, indicators and adjustments function properly.

Because these were plastic, they were not marked with an anchor stamp or star stamp to designate navy contract or army contract.

It had a wooden backplate with a plastic Kanji-filled plate which gives this hypothetical example of how to use it:

使用法

例題

觀測時刻———10時17分14秒

時差（E）——–12時4分3秒

度——-130°

ナルトキノ假定経度及地方時 ラボム

（1）A.B.C板優指標及長短遊標ラー致セシム

（2）長遊標＝廻ハシB板時目盛／觀測時刻/

10時10分二一致セシメ次二短遊標ヲ廻ハシ

C抜分目盛，7分14秒ニー致セシム

（3）B及C板零指標ョ夫々長短遊標二一致セシム

（4）2）項ト同様ノ操作二依り時差（E）12時4分3秒ョ加算ス

（5X3）項ヲ繰返へス

（6）長遊標ヲ廻ハシB板経度目盛／130一致セシム

（7）短遊標ヲA板経度目盛／1ニー数セシメC板

目盛ニテ41ラ請ミ假定経度13041ョ得

（8）長遊標／時ノ請ミニョリ地方時331得

記録

“Example

Observation time———10:17:14

Time difference (E)—–12:04:03

Degrees——–130°

Narutokino hypothetical longitude and local time Lab

(1) Align the long and short markers on the A, B and C plates

(2) Rotate the long marker to match the B plate hour scale/observation time/10:10, then rotate the short marker to match the C plate minute scale, 7 minutes 14 seconds

(3) Align the long and short markers on the B and C plates

(4) Add the time difference (E) 12:04 minutes 3 seconds using the same procedure as in item 2

Repeat item (5X3)

(6) Rotate the long marker to match the B plate longitude scale/130

(7) Move the short marker to the A plate longitude scale/1, and set the C plate scale to 41 to get the fictitious longitude of 13041

(8) Change the long marker/time to 331 degree local time

Record”

No Japanese pilot display is complete without such an instrument. These are as integral as a life jacket, goggles or boots, for without this device, no pilot would ever take off from an aircraft carrier or Pacific Island airfield.