Airbus Aircraft’s Post

Transcript

321 stop. Exactly. As opposed to flight at low speeds and more generally high angle of attack conditions, when reaching the red limit at high speed the aircraft is far from being in a danger zone. Above VMO or MMO, there is still a huge flight domain that is free of flutter. The airplane can fly up to VD or MD's, which are designed dive speeds used for the structural sizing. Limit loads are reached only with severe manoeuvres and these loads are not achieved with a simple overshoot of VMO or MMO. Finally, there is no risk of loss of control. The minimum margins between VMO, MMO and VMD are determined by the regulations. They are validated by what is commonly called the FAA dive. The dive is performed in normal law with auto thrust disconnected as follows. Determination of the thrust to precisely maintain VMO in level flight. Start a dive, such as to cross VMO at the same altitude with the same thrust and a flight path angle of -7.5 degrees. Maintain the flight path angle or full forward stick if -7. 5 degrees cannot be maintained for 20 seconds after passing VM O. Then release the stick or pull 1.5 G if the load factor is lower. Simultaneously put the engines to idle. One second later, deploy the air brakes. The maximum speed reached must not exceed VD. The same maneuver is performed between MO and MD and another one starting at the corner point VMO MMO. On aircraft without high speed protection, the difference between VMO and the maximum speed reached during the dive is usually around 50 knots. On fly by wire aircraft equipped with a hard high speed protection, the -7.5 degrees flight path angle cannot be maintained. The final part of the dive is performed with a lower pitch down attitude and therefore the speed increase is smaller. The margin between VMO and maximum speed is reduced and usually less than 35 knots on an Airbus. For a given operational speed, VMO VD can be significantly reduced with a heart protection. As VD is a design speed for the maneuvering envelope, the loads can be considerably reduced because they are a function of V ^2. It results in significant weight saving for the structure. This is the key reason for having a high speed protection. For fly by wire aircraft equipped with a high speed protection, the same type of dive must be performed with a flight path angle of -15 degrees or full forward stick if -15 degrees cannot be maintained. Recovery is initiated 3 seconds after activation of the overspeed warning. A high speed protection may be activated by a very strong gust at high altitude in specific weather conditions. If design precautions are not taken in level flight, it may lead to a level change. It is an important safety issue with RVSM operations. 1000 feet vertical separation for trajectories. This is why the tuning must reduce the probability. Of activation with strong gusts. On modern aircraft are first protection against high speed overshoot is given by the auto thrust. The thrust is reduced, possibly up to idle when approaching VMO MMO. There is an anticipation term in the computation in case of dive or high acceleration. On some airplane models, the air brakes may also be automatically extended when close to VMO MMO. As there is no risk of loss of control and even of damage to the aircraft when slightly exceeding VMO or MMO, the activation of the High Speed Protection law is delayed above V, MO and MO by around 10 knots for VM O. With this tuning associated with the Auto Thrust device, only very strong gusts can activate the heart protection. As the protection is not activated immediately when exceeding VMO MMO, there is only around 25 knots for the recovery before reaching VD or MD. A significant load factor needs to be applied at the end of the dive. The tuning ensures that all handling quality aspects are properly covered. As an example, at high altitude during the pull up, there might be interference between the high speed and the high angle of attack protections. In this case, the latter must have priority. The aircraft should come back within the VMO MMO range by itself. Stick free. Obviously, all these tuning tests are only performed after the opening of the entire flight envelope. If it appears that VMD may be overshot during the tests, immediate action is taken, engines to idle, pull up. Air brakes extension. The behaviour with very strong longitudinal gusts is checked in the development simulator. Any significant change on the aerodynamics of the aircraft necessitates a new tuning of the protection, or at least a validation of its tuning.