When the first wide body aircraft were developed, engines were not yet that powerfull. Therefore manufacturers has to use three or four engines to get enough power. Douglas, soon to merge with McDonnell to be come McDonnell-Douglas, opted for three engines. Two hang under the wings while the third is located at the base of the vertical stabiliser. Other distinctive features of the DC-10 and MD-11 are the relatively large cockpit side windows and a third main landing gear leg on most versions. 

The different versions of the DC-10 and MD-11 can be distinguished by:

• the length of the fuselage
• the presence of winglets
• the number of main landing gears
• the presence and location of a large cargo door
• the shape of the tail engine nacelle
• the presence of a refuelling boom under the rear fuselage
• the presence of cabin windows 

Both are the first pure passenger versions of the DC-10, of which the -15 has higher rated engines than the -10. They can be recognised by the absence of a third, centre main landing gear, so it has only two main landing gears. 

Being a version that could be converted between passenger and cargo aircraft the DC-10-10CF has a cargo door in the left forward fuselage, but retains cabin windows. If the door cannot be easily seen, the break in the window line is also a clue.

A DC-10-10CF, distinguished by the cargo door, many cabin windows and two main landing gears. Note the one cabin window missing in the row between the first two doors. (photo: Jon Proctor/WikiMedia)

This is a conversion of original passenger and convetible DC-10-10s and DC-10-10CFs to a true freighter. All have a large cargo door in the left forward fuselage, and metal plugs instead of cabin windows.

MD-10-10F is the designation of DC-10-10Fs with a modernised, two crew cockpit.

The increased range of the DC-10-30 meant a higher maximum take-off weight and that necessitated the addition of a third main landing gear with two wheels. It is located between the existing four wheel main landing gears. This is the main recognition point, although the wing span was also increased compared to the DC-10-10.

With the additional of an extra fuel tank in the belly the standard DC-10-30 becomes a DC-10-30ER, with extended range.

The DC-10-30 also had a convertible freighter (CF) version. It has a large cargo door in the left front fuselage, but also many cabin windows.

Converted DC-10-30s and dedicated freighter are both called DC-10-30F, although the latter are also known as DC-10-30AF (all freighter). Both have (cabin) window plugs, even the ones that have been built as cargo aircraft from the start.

FedEx also converted her DC-10-30Fs with upgraded avionics, and named them MD-10-30F.

A DC-10-30 with Pratt & Whitney JT9D engines is designated DC-10-40. These engines have a slightly larger diameter. Therefore the air flow channel of the centre, tail engine has two bulges, at the front and the back. The standard version has a centre main gear like on the DC‑10‑30.

Specifically meant for the Japanese domestic market the DC-10-40D (D for domestic) has a lower maximum take-off weight, so the centre main landing gear is not needed. For the rest it is extermally the same as the DC-10-40.

The DC-10-40D has the same wider diameter of the centre engine, but lacks the centre main landing gear. (photo: JetPix/WikiMedia)

The United States Air Force bought sixty KC‑10A Extenders as tanker and transport aircraft. They are based on the DC-10-30CF, but without cabin windows. Additionally, they have a refuelling boom under the rear fuselage.

The Royal Netherlands Air Force converted two exisiting DC-10-30CF aircraft. Unlike the KC-10s they still have cabin windows. Otherwise these KDC-10s are the same as the KC-10.

The MD-11 can be considered as the second generation DC-10. The fuselage was stretched, winglets added and engines modernised. The winglets have a big part pointing up and a small part pointing down.

The MD-11 with an extra fuel tank is marketed as MD-11ER.

The combi version of the MD-11 has a large cargo door in the left rear fuselage, behind the wing. It retains the cabin windows as the MD-11C can be used a full passenger aircraft as well. 

Behind the winglet you can observe a large cargo door, which makes this an MD-11C. (photo: Konstantin von Wedelstaedt/WikiMedia)

Besides a combi aircraft McDonnell-Douglas developed a convertible version, the MD‑11CF, that can be used as full passenger aircraft or full freighter. It has a cargo door in the left forward fuselage and the full range of cabin windows.

Both converted passenger MD-11s and purpose built freighters are designated MD-11F. All have metal plugs in place of the cabin windows, also the aircraft that were built as freighter from the start. The large cargo door is in principle in the left forward fuselage, but there are also converted MD-11Cs that retain the door in the rear fuselage.