An introduction to the Fenix A319 & A321 Expansion

As the A319 & A321 have matured nicely in testing, we felt now would be a good time to start talking in a little more detail about what we’ve been working on. Firstly, Dave, our resident aircraft history nerd, has written a little about the real aircraft, and as any good parent does when their child comes up to them with some pasta and glitter glued to a piece of A4 paper, we’re going to indulge him and put it up on the proverbial fridge. So, here’s Dave…

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Before starting this project, I naively figured the A319 and A321 were akin to the 737NG family; a bit of stretching, bit of cut-and-shunting, but otherwise the same. Turns out, there’s a heck of a lot of differences and compromises that all come together to make the piloting experience (mostly) seamless across the family. Mostly. More on that later…

Back in the 1970s, when everything was brown and yellow, and the A320 wasn’t even an Airbus, but part of the Joint European Transport programme, some bright sparks decided the 737 was outdated (I wonder what they’re thinking 50 years later) and needed some modern competition. Even this early on there were plans for 3 distinct variants: SA1, SA2, SA3. Small Aeroplane 1, Small Aeroplane 2, and Smal- what? Oh. Single Aisle 1, Single Aisle 2, and Single Aisle 3. After some faffing about with something called an A340, Airbus eventually took the reins and designated SA1, SA2, and SA3, the A319, A320, and A321 respectively.

Sensibly, Airbus hedged their bets and selected the A320 as their initial foray into single aisle aircraft. With an intended 150 seats, they figured most of their homework would apply to the 125 seat A319 and 180 seat A321. After a prolonged development period, the A320 flew in early 1987, to which airlines responded by throwing large sums of cash at Airbus pleading for them to, “go on, do another one.”

Their Shrek 2 of the series was the A321. The fuselage was relatively easy for the most part: add a couple plugs front and aft the wing and delete the small overwing exits in favour of larger ones fitted in the new plugs – though the window spacing for the original overwing exits remain. The wing, however, would be a bit more of a challenge, as early on it was realised an unmodified A320 wing would be quite compromising for performance, and limit its competitiveness with the freshly incoming Boeing 737-400, and established success story, the 757-200. Some low hanging fruit was picked off, which eventually became the cross-family ‘Lift Enhancement Package’ (we’ll get onto this later), but it was obvious more significant changes would be required. Double-slotted trailing edge flaps were designed and fitted along with some subtle changes increasing wing area by about 4%, helping a wing originally designed for a 70-ton airliner lift nigh-on an extra 15 tonnes – though this came with a penalty of additional weight and complexity. This same wing (albeit with some further flap modifications) is now working even harder on the 101-ton A321XLR, leading to take-off speeds that are just shy of tyre speed limits.

Now back to the A321CEO – the extra weight meant extra thrust would be needed to meet expected performance criteria. This was achieved on both the IAE and CFM engines through some minor modifications and an increased maximum N1. All would be well until a new noise directive due to come into effect in 2006 left the A321’s CFM56 variants just slightly over the limit, requiring some remedial action to keep it in line with the rest of the A320 family. The Acoustic Upgrade Package, launched in 2003, saw chevrons added to the exhaust nozzle and some additional noise suppression material around the reverser nacelles, brought the noise signature back by about 10dB. Airbus offered this as the Noise Reduction Package for the A319 and A320, however uptake was a statistical anomaly. The A321 would make its maiden flight in 1994, and remains a pillar of the A320 family, with the A321NEO/LR/XLR variants selling so well it may even eclipse the A320 in the future.

The A321 has seen usage on a surprisingly broad range of missions over the years, from dense regional and intercity usage to long and thin cross-continent routes that don’t have the year-round demand to justify a widebody, such as JetBlue’s nearly 7 hour Boston – San Francisco flight, or WOW Air’s 3300 mile Reykjavik – Tel Aviv. When equipped with the optional Additional Center Tanks (something we will be adding to our A321 down the line!) an A321 is capable of impressive performances including trans-Atlantic flight – though more airlines are using the 321NEO/LR now for these extended range missions.

Airbus’ Shrek 3 was to be the A320M-7, now known as the A319; a relatively easy win for the drawing office as everything bar the fuselage looked safe to reuse. Though there were some subtle changes to the A319 that had come about as part of trying to lower the A321’s pitch attitude during take-off and landing: additional strakes on the outboard engine cowling (rather than just the A320’s inboard cowling strakes), extensions to the inboard spoilers and wing trailing edge, inboard slat enlargement (pretty sure I’ve been offered this in a spam email), and some other minor aero improvements such as a tighter seal between the inboard and outboard trailing edge flaps. Now, if any of that sounds familiar to you, it’s because these modifications became the “Lift Improvement Package”, standard on all A319s and A321s, but weirdly, only an option on the A320 CEO – which didn’t see as much uptake as you’d expect, though is now standard on the A32N. This package is not to be confused with the ‘A320 Enhanced’ improvements that came in around MSN 3000, featuring a reprofiled engine pylon, wing-to-body fairing, and lower drag fuel vents.

Now, those are the improvements they made, let's talk about the compromises. When the designers realised they’d need to shorten the A320 by 7 frames (just over 12ft), they immediately ran into something, almost literally. The cargo doors. The A319 was now so short, half the rear cargo door was now also half the rear wingbox. It’s obvious Pierre in the drawing office did not enjoy this exercise of redesigning the rear cargo door to include some of the rear wingbox, because when he realised the front cargo door would also intersect the wingbox, he did something truly horrific: he shortened the wingbox by about a foot to avoid it intersecting the cargo door… but only on the right-hand side of the aircraft. This asymmetric wonder has evaded most A319 representations in Flight Simulation, until now. May you notice it on every flight. I can only presume this was such a harrowing experience for all involved that no one dared to utter, ''Pierre, what about the bulk-cargo door?”, which is entirely missing from the A319. At least they could remove one of the overwing emergency exits for a bit of weight and complexity saving. Well, until easyJet rocked up and asked Pierre if he would be so kind as to put them back. Poor Pierre.

Thrust would yet again be provided by the IAE V2500 and CFM56-5 engines, this time with a healthy derate to keep would-be 757 pilots in their place. The A319 would take to the skies for the first time in mid-1995, and launch into a market that, in the end, wasn’t really looking to replace its 120-seat 737s, BAC-111s with like-for-like, and instead mostly opted for the slightly more economic A320. However, the A319, with all the lift improvements from the A321 development, and overall lower weight, did allow it to find use on some unique routes across the world into ‘marginal’ airports. Druk Air send them into Paro, Volotea into Florence, and BA chucked an A319 into Guernsey for the lead up to the London 2012 Olympics. These days more than ever, A319s are utilised on field-limited missions that are just slightly too uncomfortable in an A320 CEO.

Now, let’s say you’re an operator with the full house, A319 through A321, what do your crew need to know? Well, the good news is Airbus worked very hard to try and keep them all as alike as possible from an operations perspective, so the list of ‘gotchas’ is quite short, but there is this annoying phenomenon known as ‘physics’ that can muddy things. Starting on the ground, taxiing a lightly loaded IAE A319 can be somewhat like walking a greyhound that just heard a car backfire: if you’re heads down in the MCDU for more than a few seconds during the taxi you’ll have gathered enough speed to practise your “V1” callout. And don’t think riding the brakes is the answer either. In fact, poor management of this on long taxis can lead to uncomfortably high brake temperatures by the time you reach the runway. Thankfully Airbus have a recommended procedure: allow the speed to increase to 30 knots, then brake back down to 10 knots. This will be the most thermally efficient way to keep the speeds managed. On the opposite end of the spectrum, when moving off after pushback a heavy CFM A321 will have you worried about blowing the ground equipment away. And on the subject of the A321, the distance you sit ahead of the main landing gear means you’ll need to be quite careful when turning on narrow taxiways; overshoot the yellow line a tad as you would with a widebody. For the most part, the rest of the flight will feel familiar. The A319 can ride turbulence really rather well, so if you’re in an A321 and hear an A319 on the struggle bus in weather up ahead, get your shoulder straps on now.

Into the descent you’ll notice a trend: the A321 doesn’t slow down, and the A319 slows down a bit too much. One would be forgiven for thinking Airbus didn’t update the VNAV calculations for these two. Once you’re lined up on final, click the AP off and start flying, the differences slowly start to come through, though not in the way you might expect. Remember how the A319 slows down too much in the descent? Well, once fully configured on final it’ll pull the Uno reverse card on you and keep hold of its speed like an ATP to a false localiser signal. Other than that, the A319 will just feel like a really light A320, and the A321 a really heavy one. But don’t let this lull you into a false sense of security; the A321 specifically requires a bit more management into the flare for two reasons. Firstly, you’ll be coming in with more inertia, so the usual “idle and pull” at 30ft isn’t going to give you enough time to arrest the descent. Instead, you’ll want to flare a little bit earlier, and a little bit less, even (the opposite is true for the A319: pull the power out later, and flare more). This leads us onto the second A321 specific behaviour: you’ll need to land tail-minded, as that extra long fuselage also means a tail-strike is at a worryingly low pitch of 9.5 degrees. Holding it off for a butter or overcorrecting for a sudden gust will inevitably lead to the FWC yelling “PITCH! PITCH!” as it frets over the possibility of something non-rubber contacting the runway.

We’re not finished yet, however. Something else to consider is braking – if the A319 is about managing the brakes on the departure, the A321 is about managing brakes on arrival. The A321 has virtually the same brakes as the A319, but lands about 15 tonnes heavier and 20 knots faster. So you’ll need more braking force for the same level of deceleration, from a higher speed. Brake temperatures in the A321 can be a challenge, especially in hotter climates with short runways; be mindful of how long you’re booked on the ground, and consider rolling to a further exit or relying on a higher reverse thrust setting to keep the brakes cool.

For me personally, when we embarked on the A319 & A321, I was dead certain I’d pretty much exclusively fly the A321, and leave the A319 for the odd easyJet flight, but having spent a lot of time flying both now I must admit I was wrong. The A319 quietly became my favourite without even noticing. The louder engine noise in the cockpit, the ability to chuck it down in some seriously tight airports, and the ease of taxiing about with a bit more speed than perhaps I should without scrubbing the nosewheel as I’m liable to do in a heavy A321. I’m not the only one either, as we track our testers we noticed an immediate flock to the arguably more popular A321, then over time, a drift towards the A319 - there is a charm to it that’s difficult to describe until you’ve done a few sectors with it. I’m really happy we’re sending out both as a single package, because I think you might just join me on team short-bus. Am I allowed to say that Aamir? Aamir…?

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We have public liability insurance specifically because Dave works here and I think you all now know why. Anyway, I think those paragraphs up there are quite handy in illustrating the differences in a rather visceral fashion. For me, the biggest difference I notice when flying any of the aircraft now is that I spend much more time with the AP switched off, as I’m always flying different airframes one after the other. It really adds some colour and variety to our product line and I’m so pleased to finally be able to share a little bit about it properly.

So, let’s begin with some details about the software package we’ve put together for you and the nuts and bolts of the last year or so of work.

To begin with - let’s talk about fuel. The A319 and A320 share fuel systems, but the A321 is markedly different, down to the type of pumps used. Airbus attempted to simplify the fuel system for the A321 by removing the split wing-tank design and using only a single, large tank per wing. In doing so, they also replaced the electrical centre tank pumps with something called transfer jet-pumps. Transfer jet pumps work by using motive flow to extract fuel from the centre tank and move it to the wing tanks - the suction in the jet pump is supplied by the wing pumps themselves in a physics based relationship and most types of jet-pumps actually have no moving parts. Now, while the older electric centre pumps could directly feed either engine - this is something the A321 can no longer do. The jet-pumps instead feed into the wing tanks now, and the wing tanks then feed the engines. This is represented by a change to the overhead fuel panel- if you look closely, you’ll immediately see how the layout has changed.

I mentioned early on in the original A320 marketing that the fuel in our simulation literally sloshes around in the tanks, and you can trigger a FUEL LEVEL LO warning with a bit of enthusiastic manoeuvring and just the right amount of fuel. This is both good and bad, because it meant we couldn’t just phone-in the job and build out the visual display changes and leave it at that, it would “break” the simulation.

This meant we had to consider how differently the jet pumps worked when there is no longer any suction provided by the wing pumps for any reason - be it electrical failure, pump failure, or anything else. The cascading order of failures is also one to consider - a failure in the tank pumps would have completely different ramifications on A320 to A321, and would require distinct consideration and actions for resolution. When something does break, and the pump is operating in an abnormal state - both are wildly different too. We needed to consider the different construction and physical capabilities of the pumps to correctly emulate the behaviour of the aircraft in responding to whatever you may choose to throw at it. An example - a failure in the electrical supply of the aircraft leaves the pumps stranded. The A320 cannot gravity feed from it’s centre tanks - that fuel is effectively unusable, but the A321’s jet pumps will continue to work off the suction provided by the gravity feed of the wing tanks as fuel moves through and a “syphon” is created, and can actually slowly transfer some fuel out of the centre and into the wings. That syphon will eventually break - and the transfer will stop - this could be due to climbing too high (creating vapour in the tanks) or if the fuel level in the tank drops too low.

Anyway, I’d love to carry on about fuel and jet-ejector-venturi pumps rather than electric feed pumps, but I need to talk about other stuff here too so we’ll move on - hopefully that gives you a quick overview of that area. A small note here: the A321 had optional ACTs (additional centre tanks) which some operators could specify for transcon missions - whilst not present in our simulation right now, we’ll be starting work on ACTs soon, and will provide it as an option later on.

So, another difference between all 3 variants of aircraft is the ECAM. Now, you may think of the ECAM as just a display - but it’s so much more. The ECAM is the central nervous system of the A320. There are thousands of pages of logic behind how the Flight Warning Computer ingests, parses, and outputs information, messages, and instructions when necessary. It’s all a delicately timed swiss-watch of a system, with thousands of sensors all over the aircraft feeding information into the interface between you and the aircraft. Everything from pneumatics, to engines, to hydraulics, to fuel, to the lavatory recirculation fan is wired up and sending information back and forth to the ECAM.

Within those thousands of pages, there are tons of messages, inputs, outputs and decision trees that are in certain cases unique to the A321, or the A319, or even the A320. Some may apply to A320 and A319, but not the A321, and vice versa. This is quite a unique thing to have to do given it is an Airbus; with the extreme degree of computerisation on board, these extra steps are critical in ensuring genuine compliance between the dozens of various computers on board. It isn’t an immediate massive change on the surface, but if you know what you’re looking for and spend some time with the product, you’ll spot (or hear… “PITCH! PITCH!”) these differences - and that’s sort of what we’re all about.

We’ve talked a fair bit so far, and I still have a lot to cover, so I’m gonna fire off some rapid ones now.

The ELAC (Elevator and Aileron Computer), SEC (Spoiler and Elevator Computers) and FAC (Flight Augmentation Computers) were all then altered for A321 and A319 conformity - things like spoiler deflection angles (including roll-spoiler relationship), inhibition rules, all included and variant-unique, as I saw a few people wondering about that specifically.

All cabin announcements now support and include the A319/A321, so type specific cabin announcements are possible (e.g. welcome on board our A319).

The brake temperature simulation natively accommodates the increased/decreased weight as the brakes themselves are relatively unchanged between the variants in reality. So, as Dave mentioned, be wary: the A321 takes no prisoners with an additional little “gotcha” of a 260 degree trigger for the “brakes hot” ECAM, as opposed to a 300 degree trigger on the A320/A319 (loops back to what I said about variant unique ECAM!)

We then return to quickly talking about data again - our flying supercomputer needed to be updated and fed with swathes of new data (and in certain cases modifications to the actual calculations/algorithms themselves) for things like green dot and VLS computation for the variants, followed by feeding it unique OPT/MAX computations for each variant - along with updating all our performance applications to provide takeoff and landing distance calculations.

Of course, the SDACs/DMCs got a little bit of love and you’ve got some little unique per-variant differences on the various lower ECAM synoptics, but elsewhere you also have things like the tail strike pitch limit indicator on the PFD, which does not exist on the A319 - and is positioned differently on the A321.

We also ensured various flap limits and v-speeds were updated in both the FMC, the FWC, and the visual model decals placed around the aircraft.

The art team are going to throw up a post soon that will have more pretty eye candy, but for now - here’s a unique little feature up on the overhead that I thought I’d show off on the A321 - the ECON FLOW pushbutton is used instead of a rotary PACK FLOW selector found on the A319/A320.

Final rapid fire before we get into the detailed stuff again, each airframe is supplied with its own unique GSX profile, so we’ve made sure you’re good to go right off the bat whether your first flight is the A321, or the A319.

Right, back into the detail stuff. It’s the last bit, I promise. Thanks for hanging in there. The flight model, the fly by wire, all that fun stuff. We’ve made some massive changes - and really, I have you all to thank for it. A while ago some of our customers complained about input lag and sent in some tickets. We did a first round investigation and observed a measurable lag between a control input and the sidestick animation - but seeing as the animation is not tied to the flight controls, it wasn't a concern. The tickets did not, however, stop - and so we took it to the second round where we started building in some diagnostics and trying to figure out what was going on. And boy, did we find out. For a not insignificant number of users, we found the time it took for the aircraft to calculate and translate control values could be as much as 300 ms. Whilst this didn't affect everyone, it was unacceptable.

However, in the process of investigating this, we realised our existing code that derived user inputs wasn't really up to our current standards, so this has been rewritten for better control translation across the board: faster, more performant, and more accurate. After this we cleaned up a tonne of fly-by-wire input, processing, and output code - it’s now running at a 100x uplift in speed. Yes, literally.

The knock-on effect of this improvement continued: we had to retune the fly by wire entirely. This is a relatively procedural undertaking as part of any control or flight model alteration, and has to be done pretty much anytime we touch this aspect of the aircraft. Normally, a retune is a marginal movement of some figures - it’s delicate. Not this time - this was sledgehammer time. The Alpha Protection PIDs are running at something like 50x lower gain value than before. The autopilot had to be completely retuned, and some code alterations had to be made to accommodate this uplift in response time and sheer speed. It’s the same for all your inputs. All tuned, airframe specific, for handflight feel at significantly lower values than before - this may not mean much to you - but the less filtering used, the more natural, direct, and connected your inputs will feel to the aircraft. These changes have brought us much closer in line to what our pilots’ ‘muscle-memory’ expected, which can only be a good thing.

Next, we turned our attention to flare - something that is a bit of a contentious topic in the community. We generally agree with the outlook that the aircraft is too floaty - but we really struggled with trying to fix it with the native controls available to us as part of Microsoft Flight Simulator’s FDE. Long story short, the nuances of an aircraft coming in close proximity to the ground and eventually making contact with it, and further still adjusting it on a per airframe basis, are beyond the scope of the existing flight dynamics engine. We decided the best course of action here was to take control of the situation as much as possible - so we’ve externalised the ground effect portion of the landing dynamics into our control. We now have reasonably granular control of how the aircraft behaves when it comes to the landing and flare portion of flight - and delivered an initial set of tuning. I eagerly await your feedback on it and we’ll make tweaks if needed.

For our customers reading this that have no intention of purchasing the expansion, no worries - you are not forgotten or unloved, you will receive these handling upgrades to your A320 on expansion launch day - our commitment is more than an open-ended promise to back-port new features “eventually”.

Finally, the flight models and engine models for all these new variants. Well, we followed the same recipe that worked for us before - we have XEM (external engine model) providing the power, with all airframes having unique engine variants and thrust ratings. You’ve all experienced XEM at this point - I’m very pleased to be bringing out more aircraft utilising this infrastructure given how reliable and accurate it’s been.

The flight models were developed much in the same process as B2’s. Our internal telemetry tool first had many hundreds of hours of data fed into it by our beta team. This allowed us to very easily compare our aircraft against real world performance data. The result is something wonderfully accurate to real world service airframes, and goes far beyond the usual “check it against the unreliable airspeed chart and call it a day” approach that is often used. Drag, lift, thrust, flap settings, geometry (duh), fuel flow, ground handling, gyroscopic precession, are all only the beginning of the parameters that were adjusted, numbering in the tens of thousands to both XEM and the FM to produce the A321 and A319 - along with us taking a close look at a few problem areas that plagued the A320 and attempting to improve on them, particularly the lack of stability when fully configured and flying close to VLS. The old simulation used to drop a wing quite frequently, even sometimes becoming jittery and nervous in the final 100ft or so of the landing, with the wings wagging about and a distinct lack of controllability if in a 30 degree bank and fully configured.

All of those issues are now solved, and you should be able to fly an extremely heavy A321 fully configured in a tight visual circuit without having a wing drop anymore. This is key because I feel it was a really sore point in our flight model before. Of course, we will once again apply these improvements to the A320 on expansion launch day.

I do feel this post has gone on too long, so for now, have a good rest of your day, and next time Dave will be along fresh out of the paintshop with some livery previews…