The first student that I ever recommended for an instrument rating checkride had to change aircraft on the morning of the test because his training airplane had a magneto problem. Another Cessan 172 was selected from the flight line. It was the same model and had similar equipment.
The student and examiner departed on a round-robin IFR flight plan that would include ILS, VOR, and NDB approaches, with appropriate holds and other tasks. As they climbed out, the tower turned them over to departure control. The student called Departure and was acknowledged. About five minutes later, Departure said, "November Five-One-Seven-Seven-Echo, climb to five thousand and turn left to a heading of two-four-zero." About a minute passed, and Departure repeated the instruction for 77 Echo.
When there was still no response, the departure controller said urgently, "Cessna Five-One-Seven-Seven-Echo, how do you read Departure?" Still no response. At that point the student said to the examiner, "Boy, when that guy finally answers Departure, he's going to be in big trouble." The examiner said, "Yes, he sure will be."
About then the student looked at the registration number placarded on the panel in front of him. It read "5177E." The student had the tail number of his training airplane in mind. With all that was going on, the student had reflexively used the old number. The student acknowledged the instructions, received a mild reprimand from the examiner and Departure, and went on to successfully complete his checkride. But he endured unnecessary stress and could have blown the test. He was rattled before he really got started.
Other differences-like digital displays on the radios, the style of push-to-talk switch, and the model of ADF-played a role in his uneasiness. The student also reported that the airplane "flew differently."
All aircraft, even those of the same make and model, are different in some way-the way instruments are arranged, the way avionics are configured, and how they handle.
It may be better to postpone the checkride until the training airplane is available. Students may develop a bond with their training aircraft and are more comfortable with it than others. I've even heard them talk to it. A training airplane can be like a favorite pair of shoes. With an uptight student, the odds of success may be improved if he or she is flying a familiar machine.