The United States is increasing Yemeni air mobility by providing money to repair and service ten Mi-17 helicopters they already have, and providing four UH-1 II helicopters (along with training on how to operate and maintain them.)
The UH-1 II is a twin-engine version of the original "Huey". Two engines made the five ton UH-1 II safer and more reliable, and it could also carry a few more passengers. Meanwhile, the half century old, single engine, UH-1 ("Huey") is fading away. Over 16,000 UH-1s were built, and over 4,000 were lost during the Vietnam war. Over two thousand UH-1s are still in service. The 4.3 ton, single engine, UH-1 can carry two crew and eleven troops, and was the first military helicopter to use gas turbine (jet) engines. This allowed a lighter helicopter to carry more weight. The UH-1 served the army for fifty years, although since the 1990s, most served in reserve units. The twin engine UH-1 was originally developed for the Canadian military, and later adopted by the U.S. Navy, Marines and many foreign countries who were willing to pay a premium for the twin engines. With various upgrades and updates, this model became the UH-1 II.
The Mi-17 is the export version of the Russian Mi-8, a twin-engine helicopter, roughly equivalent to the U.S. UH-1. But the Mi-8/17 is still in production and is the most widely exported (2,800 out of 12,000 made) helicopter on the planet. The Mi-8 is about twice the size and weight of the UH-1, but only hauls about 50 percent more cargo (about a third more than the UH-1 II). However, the Mi-8 has a larger interior, and can carry 24 troops, versus a dozen in the UH-1. The UH-1 was replaced by the UH-60 in the 1980s, while the Mi-8 just kept adding better engines and electronics to the basic Mi-8 frame. But the UH-60, while weighing twice as much as the UH-1 (4.8 tons), could carry as much as the 12 ton Mi-8. But the Mi-8 costs about half as much as a UH-60, and the larger interior is popular with many users. Russia also offers lower rates for training pilots and mechanics. This is why Yemen ended up with ten Mi-17s, but corruption and mismanagement led to poor maintenance, and most of their Mi-17s became inoperable.