I wonder if it's widely known that update 9.1 has made lots of add-on aircrafts incompatible by changing their flight dynamics in the direction of the lesser performance. For instance, fsfrance concorde no longer can climb to 50K or accelerate to M2.
Here's the response of Microsoft Support to the incident I opened on this issue.
...The basic issue you were experiencing was that after installing the update for Flight Simulator 2004, some of the third party aircrafts are unable to attain certain high height and speed. As I told you over the phone, updating the game not only updates the basic game rendering engine, it also updates the way in which the aircrafts interact with the game. It also upgrades the default aircrafts according to the needs of the updated game.
We have seen a few similar issues in the past few weeks. Troubleshooting these issues to our best possible ability hasnít given us any results. But these issues finally got resolved by installing the updated versions of these aircrafts available from the creators of these add-ons.
This situation can be compared to a cheese pizza. Previously, Microsoft was supplying a medium size plain cheese pizza, and others added a fixed amount of toppings to it (like olives, onions, etc). Now Microsoft is supplying a large size plain cheese pizza, but others are still adding the same amount of toppings to it. To get the best taste of the pizza, others should now increase the quantity of toppings that they add to the base pizza supplied by Microsoft. So basically the ball is in their court, and there is nothing that Microsoft can do about it. Only an updated version of these add-on aircrafts will help resolve the issue.