In May 1940 Lieutenant Kao You-Hsin was serving in the 21st PS, 4th PG. This unit was at the time equipped with Polikarpov I-15bis.

On 22 May Japanese bombers caught Chinese fighters refuelling at Baishi-Yi Airfield, destroying 5 and damaging 7. They managed to do this by flying close enough to Chungking to cause the Chinese to scramble their fighters and then circling out of range until the Chinese ran low on fuel and had to land. A reconnaissance plane monitoring all of this radioed the bombers to come in and attack after the Chinese fighters started refuelling.

On 26 May the Chinese warning network reported three separate raids inbound for Chungking; one group of 27 bombers passed Chang Yang at 10:09, another group of 36 bombers passing the same place at 10:44 and finally a third group of 36 passing Wu Feng at 11:20. Three reconnaissance planes were also reported supporting the bombers.
As a result of the experience from 22 May the Chinese had changed tactics. At 11:40 they scrambled two I-16s from the 24th PS, four I-15bis and four Hawk IIIs from Kwangyang-Ba Airfield. The I-16s were to look for the reconnaissance planes and the biplanes to look for the bombers while holding another group of fighters in readiness. Sure enough, the Japanese bombers circled out for range until 13:45. By then, the shorter legged Russian-made fighters had run low on fuel, but instead of landing at their home base, they diverted to other airfields to refuel. In their place, the group of fighters held in readiness was scrambled. However, the Chinese didn't quite get their timing right and the second group of fighters did not get high enough in time to intercept the Japanese. The only ones that managed to make contact were two of the four Hawk IIIs led by Captain Yuan Chin-Han, vice-commander of the 22nd PS and two I-15bis from the 21st PS led by Captain Chen Sheng-Hsing.
Patrolling at 5,000m between Baishi-Yi and Kwangyang-Ba airfields, Yuan’s group was getting low on fuel and two of the Hawk IIIs had broken off to refuel at Hsu-Ning when he spotted a group of 36 Japanese bombers dropping bombs on the Hua-Lung Bridge. Leading his wingman Lieutenant Wang Lung-Kwang on an intercept course to the south-west, Yuan caught up with the Japanese over Tachung-Ba. Attacks on the Japanese formation were made first from the right, then from the left and finally from directly below. Wang's 50-caliber (12.7mm) machinegun jammed on his third pass and he was left with to make a couple of more passes with his 7.92mm machinegun before breaking off to land at Yi-Bin Airfield. The more experienced Yuan carried on alone and soon, his shooting brought some results. One bomber was seen to fall back from the No.3 shotai with its left engine on fire. Yuan concentrated on this straggler, boring in to within 100m on his firing passes as the Japanese bomber lost speed and descended lower and lower. Breaking away in a dive after one pass, Yuan found himself so low that he almost crashed into the mountains. Frantically pulling up, Yuan repositioned himself for another pass. In the next three attacks, Yuan concentrated on the bomber's right engine and broke away only after he had also run out of 50 caliber ammunition. By this time, both engines of the Japanese bomber were smoking heavily.
During this period, Captain Chen Sheng-Hsing and future ace Lieutenant Kao You-Hsin arrived in two I-15bis from the 21st PS. Both had been descending towards Baishi-Yi to refuel at 13:45 when they spotted 27 Japanese bombers heading from the south-west to bomb the airfield. Climbing for altitude once again, Chen spotted two other formations of Japanese bombers. One (the 36 bombers attacked by Yuan and Wang) was coming much closer. What's more, there was a straggler that had fallen behind. Initially, the other Japanese bombers had slowed down to box in and support the damaged bomber. On his first pass from the front, Kao had to fly through the fire of the other bombers to get at the damaged bomber. Chen bore in from behind and managed to draw off some of the fire concentrated on Kao. Soon, the damaged Japanese bomber had slowed down so much and descended so far that the rest of the Japanese formation had to leave it behind. After Yuan broke off, Kao followed the Japanese bomber until it crashed into the mountains while Chen covered him from above.
Credit for the kill was awarded to Yuan, Kao and Chen.

25 G3Ms from the Mihoro Kokutai attacked Lanzhou on 22 May 1941.
At 10:20, the 4th PG ordered its aircraft to fly to dispersal strips. Liu Chi-Sheng, Squadron Commander of the 24th PS, led seven I-16IIIs towards Wu Wei with an SB-III flown by Yang Chung-An, Squadron Commander 9th BS, providing navigational support.
One of the I-16III could not retract its gears and diverted to Hsi Ku Cheng Airfield at Lanzhou. At 11:02 the remaining I-16IIIs found bad weather in the vicinity of Wu Wei and diverted to Chung Chuan Chun Airfield north of Lanzhou.
Shortly after 12:10 when everyone had landed, the 25 Mihoro Kokutai G3Ms passed overhead. Kao You-Hsin had not yet switched off his engine and he took off, anticipating that the Japanese would turn back to attack the Chinese planes on the ground. Sure enough, nine bombers returned flying at 5,000 meters and were attacked by Kao. The first pass was made from the left side, followed by a pass from the front. Kao fired from 400 meters and dived under the formation after the pass. Two G3Ms; the leader and no. 2 of the 3rd shotai, were seen to emit smoke. Kao made three more passes from the side disrupting the Japanese bomb runs. With the precious time bought by Kao’s attack, the Chinese ground crews were able to get the remaining I-16IIIs started. All five remaining fighters got away with only one of them suffering damage from the fragments of 45 bombs that the G3Ms dropped on the airfield.
Kao broke off after firing 600 rounds, one of which holed a propeller blade due to a malfunctioning synchronization gear.
One G3M, flown by Lieutenant Shin-Taro Hashimoto, went down between Hai Yuan and Ching Yuan. All onboard were killed.

Kao You-Hsin ended the war with 1 shared biplane victory and a total of 8.

Claims:

Kill no.	Date	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1940
	26/05/40	1	Enemy bomber	Shared destroyed	Polikarpov I-15bis		Chungking area	21st PS
	1941
?	22/05/41	1	G3M (a)	Destroyed	Polikarpov I-16		Chung Chuan Chun Airfield	21st PS
	22/05/41	1	G3M (a)	Damaged	Polikarpov I-16		Chung Chuan Chun Airfield	21st PS

Biplane victories: 1 shared destroyed.
TOTAL: 8 and 1 shared destroyed, 1 damaged.
(a) Claimed in combat with G3Ms from the Mihoro Kokutai. One G3M, flown by Lieutenant Shin-Taro Hashimoto, went down between Hai Yuan and Ching Yuan. All onboard were killed.

Sources:
Information kindly provided by Raymond Cheung.