photo from a year ago
Because Y20 is not in the same class as C17. There are a lot of equipments can be fit into C17 but not in Y20, even their load capacity is not that far apart.Why bother? There are very very few things a C5 sized aircraft can do that a C17 sized one cannot.
The U.S. has modernized its C5s but only a small number and this was done mostly to supplement the C17 fleet.
the Russians have maintained a number of AN124 but mostly because they don’t have the capability to replace them.
from the practical perspective building out more Y20
The Y-20 and C-17 contrast with each other more in terms of cargo compartment size than payload capacity.Because Y20 is not in the same class as C17. There are a lot of equipments can be fit into C17 but not in Y20, even their load capacity is not that far apart.
1. Il-76: 20.00m long x 3.45m wide x 3.40m high.
2. Y-20: >20.00m long x ~4.00m wide x ~4.00m high.
3. C-17: 25.96m long x 5.49m wide x 3.96-4.10m high.
And by using these PLA land vehicles to represent the different vehicle size categories for ease of comprehension:
1. Small-size: CSK181 MRAP, which is 5.65m long x 2.12m wide x 2.38m high.
2. Medium-size: VN1 APC, which is 7.80m long x 3.00m wide x 3.20m high.
3. Large-size: HT-233 radar vehicle, which is 15.00m long x 3.05m wide x 3.95m high, plus HQ-9 launcher vehicle, which is 13.00m long x 3.05m wide x 3.84m high.
Continuing by doing some rough Tetris, and here're the results:
1. For small-size (CSK181):
- Il-76 can carry 3x;
- Y-20 can carry 3x; and
- C-17 can carry 6x (2 rows of 3x each).
2. For medium-size (VN1):
- Il-76 can carry 3x;
- Y-20 can carry 3x; and
- C-17 can carry 3x.
3. For large-size (HT-233 and HQ-9 launcher):
- Il-76 can carry none (too big);
- Y-20 can carry 1x; and
- C-17 can carry 1x.
So, here's the analysis:
1. While the Y-20 certainly can carry 6x CSK181 based on payload weight capacity alone, yet the smaller width of the Y-20's cargo compartment compared to the C-17's means that those extra payload weight capacity available for the Y-20 is wasted;
2. The Y-20 is able to carry the HT-233 and HQ-9 launcher, but only barely when taking height into account; and
3. Due to the insufficient height of the cargo compartment of the Il-76, the payload weight capacity of the Il-76 means nothing as it is incapable of fitting both the HT-233 and the HQ-9 launcher inside of it.
So there's that.1. C-5: 37.0m long x 5.8m wide x 4.1m high.
2. An-124: 36.5m long x 6.4m wide x 4.4m high.
There are a few cases when you would need a C-5-like aircraft to airlift something. But those are quite rare. Here are the photos of a CH-53E, a special ops boat and a fighter fuselage getting cargoed.The Y-20 and C-17 contrast with each other more in terms of cargo compartment size than payload capacity.
I made a post in this thread sometime ago, which compares the cargo compartment capacities for the Y-20 and C-17 through using several commonly carried (or expected to be commonly carried) land vehicles of the PLA by these airlifters (alongside the Il-76):
If anything, this shows that while carrying small-sized land vehicles can be a greater disadvantage for the Y-20 vis-a-vis Y-20's smaller cargo compartment compared to the C-17 - The disadvantage gradually diminished with medium-sized and large-sized land vehicles, as the Y-20 and C-17 broadly carry the same number of land vehicles in both these categories.
(In fact, the C-17 slightly loses to the Y-20 in terms of cargo compartment height due to its wing box which protruded into the cargo compartment.)
Therefore, if anything - China's next-gen large airlifter should focus on the suitable expansion of the cargo compartment dimension compared to the Y-20, with particular focus on the width and height of its cargo compartment in order to fit the wider and taller PLA vehicles that are expected in the future (e.g. PLA's command, radar & sensor, and launcher vehicles for mid-course/terminal ABM/AHM system).
In the meantime - While developing a very large airlifter in the C-5/An-124 category certainly does have its own merit, yet as @TerraN_EmpirE has pointed out above, the merit may be rather limited in scope and potential to become truly justifiable for R&D resources to be dedicated towards it, at least in the short-to-medium term. That's probably also why both the US and Russia has no solid plans to develop direct successors to their C-5 and An-124, respectively.
In fact, airlifters in the C-5/An-124 category don't have significantly higher cargo compartment cellings than airlifters in the C-17/Y-20 category, while having a wider cargo compartment than the C-17 isn't as demanding either:
So there's that.
if you remember, Chinese almost finalized the main assembly line of AN-225 to build second unfinished aircraft in 2016.. first flight was expected in 2019... LOOLPersonally I can't rule out China developing a super heavy lifter. Yet, I don't think it is an urgent requirement. If there is an ongoing project, it is very likely a low priority project. PLA has other more pressing needs which it would rather to spend its resources on.