In a recent study AeroAnalysis had a look at how many McDonnell Douglas MD-11 aircraft are still flying around and when most aircraft are up for replacement.
What makes things rather difficult is that it remains mostly unknown which aircraft retires at what age. FedEx initially seemed to target the MD-11s to be taken out of service at 30 years, but seemed to accelerate this phase out as demand for air cargo transport remained weak. However, we have recently seen some recovery in the market, which could push that retirement age back to 30 years. In the past, freighter airlines have been taking their aircraft out of service at the age of 40. That is something that we will not see happening, since airports have implemented regulations on emissions and noise levels, and Boeing has been working with airlines to make freighter purchases more worthwhile. So that upper limit for the retirement age of the MD-11 also should be at 35 years.
Subdividing the FedEx and UPS fleets into blocks of 5 aircraft, each with an average age, we get the following overview of aircraft reaching either the 30-year mark or the 35-year mark:
What we see is that until 2030, all 122 aircraft in active service will have reached the age of 30. If that indeed is the retirement age, then replacement should start in 2021 and peak in 2023-2024. If the retirement age is more towards 35 years, then this peak is in 2028-2029. Either way, there seems to be a replacement cycle starting in the next decade and the Boeing 777F should be the preferred aircraft to replace the MD-11F.