India claims that its T-90 deployed in Ladakh will absolutely dominate against Type-15 light tanks. On paper T-90 has thicker armor and much bigger gun. So, why did China design a light tank for mountain warfare? How advantageous is it to have a 30 ton tank in a ladakh Scenario. How would Type-15 deal with fighting a T-90 which will outgun it?
Light tanks aren't in the same class as MBTs, nor do they fulfill the same roles. Light tanks tend to be used more to exploit their high maneuverability and terrain access in support of infantry or in conduct of reconnaissance. Moreover, they tend to be worse armored, to carry worse firepower, or both.
When it comes to T-90 vs ZTQ-15, it really depends on the scenario. If the T-90s are doing an armored thrust, the ZTQ-15 can attempt to flank, but otherwise can't really do much about it due to its thin armor and underpowered main gun. It can attempt to launch gun-launched ATGMs, but there's travel time and guidance needed for the gun-launched ATGMs.
On the other hand, if the ZTQ-15s are exploiting terrain the T-90 can't use, i.e, the ZTQ-15 is attacking from an area the T-90s couldn't possibly reach, the ZTQ-15s could hit the T-90's side armor.
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In either case, the ZTZ-99 has almost the same power to weight ratio as the ZTQ-15 and is deployed to the border region. And the Chinese have IFVs armed with modern tank-destroying missiles, light vehicles with tank-destroyer load-outs, attack helicopters with ATGMs, and anti-tank artillery munitions in the area.
If a ZTQ-15 is going to be called to destroy a T-90, something has gone horribly wrong for the Chinese or the Indians--either the T-90 is vulnerable enough to be a target of opportunity for the ZTQ-15s, or the ZTQ-15s have been ambushed or there's no other tank support available at the location.
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Also, to quantify the claims that the T-90 is engine-screwed at altitude, Leh is about 11000 feet. That translates to about a 33% reduction in engine power if the T-90 doesn't have a supercharger or some other way to increase airflow. But the T-90MS has a supercharged engine, and if it's good for 6 psi of boost the T-90MS is basically running at the equivalent to sea level at 11000 feet.
The main mobility advantage of a ZTQ-15 would be through its lighter weight and higher power-to-weight ratio; i.e, the ZTQ-15 can go places the T-90 can't go and it can climb slopes more effectively than a T-90.