Armour Plate on the Ki-61 Hien

One of the enduring myths of the Pacific War is that Japanese pilots were not protected by armour plate. In fact the Army Air Technical Research Laboratory at Tachikawa began investigating the practicality of armour protection during the Sino-Japanese war of 1937 and began making serious efforts to develop it during the Russo-Japanese conflict of 1939 when downed Russian aircraft were found to be equipped with pilot protection in the form of armour plate. With the outbreak of the Pacific War the requirement for pilot protection was deemed urgent following examination of the installations in captured B-17s. The Laboratory was charged with pursuing these requirements on the following basis:-

1. To develop special steels for armour, especially substitute alloys for nickel and molybdenum which were in critical supply;
2. To develop superior heat treatments;
3. To develop composite armour sheet with double or triple layers of steel arranged at optimum distances;
4. To develop new and improved methods of surface hardening; and
5. To develop armour sheet that could defeat explosive (cannon) shells.

At the beginning of the war the Japanese used combinations of nickel, chromium and molybdenum, manganese,  chromium and molybdenum, chromate and molybdenum, or silicon, manganese and chromium but a shortage of nickel forced them to resort to silicon, manganese and chromium exclusively later in the war. They were unable to develop effective lightweight duralumin-based armour plate because of the increasing shortage of aluminium.

Ki-61 armour plate (red) as shown in the aircraft maintenance manual

In the Ki-61 Hien the armour (shown above from the maintenance manual) consisted of two plates to protect the head and back of the pilot. In the manual it is described as "resisting (or defending against) bullets steel plate" (防弾鋼板) rather than the expected "armour plate" (甲鉄板 - kabuto teppan). One approximately triangular-shaped sheet was bolted to the inverted 'T' of the rollover pylon behind the pilot's head and a larger rectangular plate to the back of the seat. The plates were usually painted matt black but degraded to a dark grey appearance over time. In some cases the armour plate was removed (for example in Shinten Seikutai air-to-air ramming aircraft) and as a salvageable commodity was usually cannibalised from wrecked or abandoned aircraft. It is therefore not always immediately recognisable in photographs of preserved or wrecked aircraft. It is presumed from the photogrphic evidence that the pilot's head cushion could be incorporated with or without the armour plate but the exact details of installation are obscure.

Ki-61 with head armour installed

The armour plates were of 10mm thickness on the Ki-61-I Ko, installed from aircraft S/N 113 onwards (August 1942).  With the Ki-61-I Otsu the plate behind the seat was increased to 12mm thickness from S/N 542 and from S/N 578 the head protection was increased to 16mm thickness (October 1943). This remained the standard installation to the end of the line.

Ki-61 without head armour - this is a Shinten Seikutai aircraft of the 244th Sentai showing a ramming victory

American intelligence documents stated that the armour on Hien tended to shatter if subjected to more than one .50 calibre hit. It might be that the quality of manufacture was inconsistent but Tachikawa tested the silicon, manganese and chromium 16mm sheet as proof against 13mm to a velocity of 710 m/s and against 20mm to a velocity of 520 m/s and to 37mm explosive shells at a velocity of 740 m/s, presumably using Japanese weapons. 

Model kits of Hien have approached the details in different ways but they are perhaps more pertinent to the larger scale models. The Hasegawa 1/32nd scale kit (above) has the armour plate as a separate piece K10, so that it is possible to depict an aircraft without the head armour installed, although versions of this kit where that is likely do not make the distinction. In this kit it is suggested to paint the armour plate in the cockpit colour. The Hasegawa 1/48th kit (below) has the inverted 'T' pylon and armour plate as one piece C13 so some adjustment will be needed to depict an aircraft with armour removed - although it is correctly called out as being matt black. 

The Fine Molds 1/72nd kit (below) also has the inverted 'T' pylon and armour plate as one piece, this time incorporating the whole seat back as part B13 but is correctly called out as matt black.

When building a model of Hien, especially in the larger scales, it is a detail worth checking whether the subject aircraft had the head armour fitted, if known, and to represent it accordingly.

References:- 

'Armour of Japanese Aeroplanes', BIOS/JAP/PR/894, which reproduces the 31 October 1945 report prepared by Cdr Sheldon W Brown of the USN ATIG from the information of Maj Taneo Koinumaru of Army Air Technical Research Laboratory, Tachikawa.

Type 3 Fighter Maintenance Manual.
'Ki-61/Ki-100 Serial Numbers, Dates of Assembly & Characteristics' by James I Long, Airtell Publications & Research Service, 1999.

Image credits: Heading diagram from Performance and Characteristics Data Japanese Aircraft ATAD # T-1 of May 1944 OP-16-V-T Revised Edition, Air Intelligence Group Division of Naval Intelligence, Office of the Chief of Naval Operations, Navy  Department; Ki-61 Maintenance Manual diagram re-traced by author; Photos from 'I J Army Kawasaki Type 3 & 5 Fighter', Model Art # 428 (© 1996); Kit instructions © Hasegawa Corporation and Fine Molds Co. Ltd.