Die Ki-84 gelangte gegen Ende des 2. Weltkrieges zum Einsatz und zeigte sich schnell den amerikanischen P-51 und P-47 überlegen. Sie war wendiger und hatte außergewöhnliche Steigleistungen.

Obwohl sie erst ein Jahr vor Kriegsende in den Truppendienst kamen, wurden die Hayate (Codename ‚Frank‘) in großen Stückzahlen gefertigt und waren trotz Triebwerksschwierigkeiten einer der gefürchtesten Gegner der Alliierten im Fernen Osten. Einige Male kam die einsitzige Maschine als Sturzkampfbomber zum Einsatz, wobei sie unter dem Tragflügel bis zu 250 kg Bomben aufnehmen konnte, aber einen wirklichen Namen machte sich die Hayate als Jagdflugzeug. Der Prototyp flog im März 1943; im August, fünfzehn Monate nach Konstruktionsbeginn, kam die Ki 84-1a in die Serienfertigung.

Triebwerk: 18 Zylinder Motor mit 1900 PS
Höchstgeschwindigkeit 624 km/h
Spannweite 11.26m; Länge 9.91m; Höhe 3.61m
Dienstgipfelhöhe 10500m
Reichweite 2900 km
Bewaffnung: zwei 20mm Typ 5 Kanonen und zwei 12.7mm Typ 103 Maschinengewehre; bis zu 500 kg Bombenzuladung.

"Forget it - it's a Frank." It is said that this comment was made frequently by USAAF personnel watching radar screens on Okinawa in the closing weeks of the Pacific War.  It was customary to watch for a contact to appear and then to scramble P-51 Mustangs to intercept the enemy aircraft.  But when the blip was moving so fast that it was inferred to be one of the advanced new Japanese Hayate fighters it would be assumed that the P-51s would stand no chance of catching the intruder.

Generally regarded as the best Japanese fighter of World War Two, the Hayate ('Hurricane') was nonetheless not without its problems.  Much of its superlative all-round performance stemmed from its extremely advanced direct-injection engine, the Army's first version of the Navy NK9A.  Yet this same engine gave constant trouble and demanded skilled maintenance.

T. Koyama designed the Ki-84 to greater strength factors than any previous Japanese warplane - yet poor heat-treatment of high-strength steel had the consequence that the landing gears often snapped. Progressive deterioration in quality control meant that pilots never knew how individual aircraft would perform, whether the brakes would work, and even whether - in attampting to intercept B-29 Superfortresses over Japan - they would be able to climb high enough.

Despite these problems the Hayate was essentially a superb fighter - a captured Ki-84-1a was to outclimb and outmanoeuvre a P-47 Thunderbolt, and a P-51.

The first batches were sent to China, where the 22nd. Sentai, when equipped with the new fighter, were able to fly rings  around Chennault's 14th. Air Force.

The 22nd. Sentai was later moved to the Philippines, where problems overtook them, with many accidents and shortages and extremely poor serviceability.

Frequent bombing of the Musashi engine factory, and the desperate need to conserve raw materials (the shortages resulting primarily from the American submarine blockade) led to various projects and prototypes made of wood or steel.

Total production of the Ki-84 reached 3,514.