Airbus - confusion over VAPP / GD speed and Approach Phase
 

It seems like 25% of the pilots I talk to don't understand what managed speed the Airbus goes to when you activate approach in the MCDU if clean configuration. Some say Green Dot, some say VAPP.

To add to the confusion, manufacturer pubs and ground school powerpoints don't talk about it much either. Even the FCOM is lacking on this topic.

Airbus: https://skybrary.aero/sites/default/...shelf/4071.pdf


https://cimg1.ibsrv.net/gimg/pprune....cc62849dab.png

This site (indeed Flaps-1 mentioned) makes NO mention of Activating approach while clean, and instead only talks about doing it in Flaps-1: https://www.smartcockpit.com/docs/Ai..._Arrival_1.pdf


https://cimg0.ibsrv.net/gimg/pprune....c2c9c850f9.png

villas and others, how do you guys explain Approach Phase and speeds. "Match the flap handle" ? etc ?

Thank you

The way I have seen it works better with students is having them think in terms of "what for", why that button is there and why we need it.

Ask them what they are going to do when they fly the airplane into the approach plate.

Make them realize, through their own thinking, that you need the airplane to bank angle 25 degrees in order to remain in the protection areas of the approach, or the holding, and you need to bank while maintaining the minimum legal margin over the stall speeds (you may introduce the definition of Characteristic Speeds here).

Let them reach the conclusion that the legal margin over the Vapp speed is much lower (when you land you need to be able to stop the airplane on a finite runway) than over the maneuvering speeds (you just need the airplane to sustain some load factor, nothing more), that is why they'll see different margins over Vls in each flaps configuration.

And then, and this part is tricky, give them the chance to understand that, while the angle of attack at each one of those speeds happen (maneuvering and Vapp) is the same, the IAS at which it happens will depend on the WEIGHT. And such, instead of having to recalculate each set of speeds in flight (let's say, when you are in the holding, waiting to start the approach, your set of speeds while get lower with each ton of fuel you burn), you can just activate the Approach Phase in the the PERF PAGE, and voila, the FMGC just gave you the set of speeds you need to fly, to maneuver the airplane through the approach chart, it will calculate them for you, accurate to your actual GW, and with a Vapp that follows the weight of the airplane when it lands, following the FOB predicted.

So, the answer is - and I have found that they understand this term better - the Approach Phase is a energy managing tool that will allow you to get the set of speeds you need to 1) fly the airplane through the approach plate and 2) land the airplane on the runway, by giving an instant calculation of those speeds following the GW at the moment of activating it.

Many FO's I fly with now activate the Approach Phase as soon as ATC gives a speed reduction, because the associate it with "reducing speed for the approach", showing that they haven't been explained what is the internal process of the FMGC.

As an epilogue, you can also explain them that, depending on the FMGC version, MSN, etc... some airplanes with calculate the set of characteristic speeds as a figure from the expected GW (following the EFOB at the time of the approach), while other airplanes will calculate the value of such speeds with date from the FAC, that is, the value of the speeds will constantly be changing when the fuel burns and the GW gets lower. But despite this, they should always follow the same procedure: let the Approach Phase autoactivate, or activate it manually when you start your approach, but not earlier, to make sure your characteristic speed are as accurate as possible.

It doesn't reduce speed to GD or Vapp, looking at it from that point of view makes it more confusing, me thinks.

It's pretty simple I thought. You activate the approach phase when you want to reduce speed for the purpose of configuring for the approach. The magenta speed target will go to Vapp BUT the actual speed target is the greater of the magenta bug or the manoeuvre speed, GD or S or F, until you're in the approach config.

Personally I leave the approach phase to be automatically activated by the DECEL waypoint unless I want/need to get flap out earlier. Depending on the energy state of the aircraft, I may select speed towards GD until the passing the DECEL waypoint if I think there will be a brief thrust increase between levelling and passing DECEL. I don't activate the approach phase just because of ATC speed control because, why? If the PM looks bored I might ask them to activate the approach phase prior to the DECEL waypoint.

My main consideration when activating the approach phase early is that I'm doing it to avoid a situation where I've selected Config 1 in selected speed then managed the speed and have it rocket back up to 250 knots.

If you activate approach phase while clean the system will target green dot. It won't go below that until you select flap.

I usually activate approach, if it hasn't activated automatically, the moment I want to reduce speed below 250kts. Why? So I don't forget.

The document you have referenced is incorrect, that was not published by Airbus.

Refer to FCOM DSC-22_30-60-20 "Managed Speed/Mach Target" for the answer, it's in black and white 👍🏼

The relevant part to this question states that once the approach mode is activated on the FMGS (or a number of other relevant criteria), then the speed target is:

The limiting speeds are not target speeds, they are exactly that, limits. The aircraft will only reduce down to that limit until the limit is removed and it can reach it's target speed. Similarly, the aircraft will pitch to not exceed Vfe, when accelerating to enroute climb speed, if flaps are still extended.

excellent, thank you