Trijets Are Bad, Actually
a brief exploration of why it's not the best idea in the universe to stick a jet engine directly through one's tail
As promised: my thoughts on triple-engine jetliners, let me show you them.
These days trijets are used for cargo (and some tiny obnoxious private aircraft), but from the 60s through the 90s — peaking in everyone’s favorite decade, the 80s — the trijet was a common and popular type of commercial passenger jetliner. The Lockheed L-1011 TriStar, McDonnell Douglass DC-10 and its stretched version, the MD-11, and the regrettable Boeing 727 are some of the best known, but you shouldn’t forget the Tupolev Tu-154 or the Hawker Siddeley Trident, which at least has a pretty awesome name.
Having three engines did confer some advantages in an era where the engines themselves were nowhere near as powerful as today’s high-bypass turbofans: trijets were cheaper and more efficient than four-engine aircraft like the 747, but were still exempt from twinjet regulations that insisted their flight path be restricted such that they were never more than one hour away from a diversion airport if one engine failed. Also, having three engines meant you could put the wings farther back on the fuselage, which both increased the aircraft’s fuel efficiency and enabled better positioning of the main entrance/exit doors to make it easier to get people on and off the thing in a hurry.
There are two main subspecies of trijet: the S-duct and the straight-through. S-duct aircraft have the number 2 engine within the aft fuselage, with its exhaust right at the back where normal planes’ APU exhaust is located, and the air intake from the base of the tail directed down a vaguely S-shaped duct (L-1011, 727, Trident, etc), whereas straight-through designs have the engine just stuck right through the base of the tail (DC-10, MD-11). It is the latter that strikes me as a Really Bad Idea, rather than just a Bad Idea, and now I will tell you why.
There is no way in hell a human pilot, even a truly beefcake example of the species, could ever exert enough force on the controls of a commercial passenger aircraft to actuate the flight control surfaces on their own, so these planes have hydraulic systems that translate the control input from the pilot to actuators that do the physical moving. This means there’s a bunch of very important hydraulic lines running through the plane, and a lot of them happen to work various bits of the tail. (The tail is crucial. You need all the bits of it in order to fly the aircraft. For an example of what happens if you are missing a significant chunk of the tail, see JAL 123.)
So we have a vital part of the plane that contains a lot of equally vital hydraulic lines. Great! Let’s stick a jet engine through it, because jet engines have never been known to suffer catastrophic fan-disk failure sending razor-sharp shards of metal slicing through anything within range! What could possibly go wrong?
United Airlines Flight 232: “Hold my beer.”
UA 232, often referenced as “the Sioux City crash”, is both an example of bad luck and amazing piloting. On July 19, 1989, a DC-10 on its way from Denver to Philadelphia via Chicago suffered what the NTSB describes as a catastrophic failure of the tail-mounted engine number two, resulting in the separation, fragmentation, and forceful discharge of stage one fan rotor assembly parts: i.e., the tail engine shit the bed in an explosive fashion, comprehensively destroying the hydraulic lines routed nearby. This meant not only losing the tail control surfaces, but in fact all the control surfaces. The only control the crew had left was thrust on the two remaining engines, and the fact that 184 of the 296 people on board survived the subsequent crash landing is due to the extraordinary efforts of the four people up front trying to drive the thing: Captain Al Haynes, first officer Bill Records, flight engineer Dudley Dvorak, and completely coincidentally DC-10 check airman Dennis Fitch, a passenger who had read about JAL 123 and done some simulator experiments in controlling an aircraft using engine thrust only. Any competent editor would nix the idea of someone like Fitch happening to be on board during this particular emergency because it’s just too unbelievable, but reality — as has been noted — unfortunately lacks competent editing.
To be fair to the trijet I will also briefly touch on an incident in which an L-1011’s third engine came in super handy: Eastern Air Lines Flight 855. Due to shitty maintenance, all three engines ran out of oil on the way from Miami to Nassau, but the sequence in which they did this is important: the tail engine, #2, showed an oil warning light first and was thus shut down, leaving the plane to fly on its other two engines. Except then #3 and #1 flamed out, leaving the plane to glide.* They got all the way down to about 4,000 feet above the sea before they were able to start #2 back up on the third try. Turned out there was just enough oil left in it to get them the rest of the way back to Miami and execute a one-engine landing. (I think I read somewhere that #2, unlike the other two, was just fine after some TLC and went on to fly several thousand more cycles, which is pretty cool.) Had that been a twinjet, everybody on board would have been fish food in short order, so the L-1011 gets some love from me.
Next week: The Pitot-Static System Works Better Without The Wasps :)
*you have no idea how much I wanted to use the Python “not so much fly as plummet” line, but it’s just not accurate: airliners do not fall out of the sky like sheep when they lose their engines. This is why the Gimli Glider was a thing.