Disclaimer:
▪︎This content is Not an official document and does not represent the views of Airbus or any other aviation authority.
▪︎The information provided may be incorrect or misinterpreted and should not be relied upon for decision-making.
▪︎Always refer to official documents and consult with a qualified aviation professional before making any decisions based on the information provided in this blog post.
▪︎The information provided in this blog post is based on personal study and review.
Even though Vapp is consistent with the FMGC speed displayed on the PFD, there might be a difference between the VLS Speed on the PFD and the speed calculated by the FMGC.
According to the Airbus Flight Crew Operating Manual (FCOM), the Flight Augmentation Computer (FAC) is responsible for computing the VLS speed. However, this calculation can vary depending on the specific aircraft type, despite all being A320 aircraft.
In some A320 aircraft types, particularly the A320-214(some of it), the FAC computes the VS1G speed using inputs such as current angle of attack, speed/Mach, altitude, thrust, and center of gravity (CG). As stated in the FCOM, the normal accuracy for VLS in CONF FULL configuration is approximately ± 3 knots, resulting in a slight discrepancy.
In other A320 aircraft types, the FAC calculates the operating speeds based on the gross weight entered in the Flight Management System (FMS). Consequently, the operating speed displayed on the PFD and the MCDU remains consistent and accurate.
However, in these type of A320 aircrafts, the FAC also independently computes its own gross weight and CG based on aerodynamic data. This serves as a backup for speed computation in the event of a dual FMS failure. Nonetheless, the FAC does not utilize the computed VLS speed to display it on the PFD.
As shown in the following pictures, even though Vapp is the same at 136 knots on the FMGC and PFD, upon closer inspection, the VLS values differ. The FMGC indicates VLS as 131 knots, while the PFD displays VLS as 127 (or 126 knots), resulting in a VLS+10 knots, according to the PFD's calculation.
It is to note that the picture situation does not include any headwind component, so GS mini is not taken into consideration.
Sometimes, in situations like this, when I handle the aircraft, I experience a peculiar sensation. It feels as if I have added extra knots to the Vapp speed.
During such instances, I am uncertain which VLS speed is the correct one. Even though it may be considered normal, it is advisable to be mindful that it can lead to different handling sensations in the aircraft.
Disclaimer:
▪︎This content is Not an official document and does not represent the views of Airbus or any other aviation authority.
▪︎The information provided may be incorrect or misinterpreted and should not be relied upon for decision-making.
▪︎Always refer to official documents and consult with a qualified aviation professional before making any decisions based on the information provided in this blog post.
▪︎The information provided in this blog post is based on personal study and review.
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