Most light airplanes fly perfectly well with a door open (or even removed altogether, as AOPA routinely does for photo shoots), and most students are presumably told this during training. Some of them seem to forget that message afterwards, though.
About a hundred accidents over the past 20 years were directly attributed to doors that opened unexpectedly. In most cases, the pilots either got distracted at a critical time or lost control of the airplane while trying to wrestle the hatch back into place. There have also been a handful in which the nose baggage door of a larger airplane popped open during climb-out; at low airspeed and high angle of attack, the additional drag and uncommanded yaw can create serious control problems. This was a factor in, among others, an Alaska Navajo accident that killed six and an Aztec crash in Texas with four fatalities, not to mention accidents in a Cessna 421 and a Beech Duke that caused serious injuries to everyone on board.
Nose baggage compartments aren’t a concern in most training aircraft, of course. The typical accident sequences involve a cabin door that pops open during the take-off roll or initial climb. Rather than aborting the take-off (if distance permits) or flying the pattern and securing the door after landing, the pilot tries to muscle it shut while the airplane’s moving. Left to its own devices, the craft goes off the side of the runway, pitches up and stalls, or banks ever more steeply until it hits the ground. Next most popular, if that’s the right word, are accidents in which the pilot got the airplane back around the pattern only to forget to pay attention to the actual landing—or to extending the gear first. The latter led to the complete destruction of a Piper Comanche that caught fire after the gear-up landing, though fortunately the pilot escaped.
Some interesting variations arise from the quirks of particular models. Piper Cherokees and Beech Bonanzas only have doors on the co-pilots’ sides. Solo pilots trying to reach across find they have no leverage with just the right arm; a serious effort requires twisting the upper body in a way that’s apt to result in some unintended control inputs. Cirrus equips both sides of the cockpit with gull-wing doors that look swoopy but seem particularly inclined to open of their own accord—sorry, guys, but it’s true—with a bang loud enough to make you wonder which crucial piece of the airplane just broke. The big windows in Cessna singles make it especially tempting to grab the door and see if you can pull it closed, but it’s worth taking the time to drive the point home to your students: Struggling with a door while you’re trying to fly the airplane is nevera good idea.
Like a gear extension failure, an open door in flight isn’t a real emergency, but it’s possible for the pilot to create one while trying to fix things. It’s noisy, it may be cold, and in IMC it’s liable to be wet and generally unpleasant. In other words, it’s a nuisance, not a problem, nothing close to good reason for jeopardizing positive control of the aircraft.
Many shrewd instructors find a way to have their primary students take off with a door open during a dual lesson. Besides proving that the airplane does still fly, it gives the student valuable real-world experience in the discipline of ignoring distractions while continuing to aviate. It might also be useful to have the instructor fly the airplane, deliberately take off with the door open, and let the student try to close it in flight, because the hard truth is that most of us can’t do it, even with both hands. The lower pressure in the slipstream creates enough suction to hold the door an inch or so from the frame, while the increased drag makes it next to impossible to open it far enough to slam with any authority. The lesson that you can’t close the door if you try neatly reinforces the primary lesson that you shouldn’t try to begin with.
This is not recommended, however, with hinged bubble canopies. Those that hinge in the front can still be rattled loose by the slipstream, while those that hinge at the back may flip open far enough to block the flow of air across the tail. Once it breaks off, either style is liable to hit the tail with enough force to damage or destroy control surfaces at the moment student and instructor find themselves in hurricane-force winds. Pilots who fly these models need to be particularly scrupulous about making sure the latches are secure before take-off—and if you’re in the market for a new trainer, it’s one more point to consider.