Airbus and Boeing Juxtaposed

It turns out that much like its 737 MAX counterpart, the Airbus A321 NEO also has a pitch up issue.

There is an important difference though, Airbus actually spent the money to make sure that was well tested and thoroughly redundant, while Boeing did it on the cheap:

The European Aviation Safety Agency EASA has issued an Air Worthiness Directive (AD) to instruct operators of the Airbus A321neo of a Pitch instability issue.

EASA writes “excessive pitch attitude can occur in certain conditions and during specific manoeuvres. This condition, if not corrected, could result in reduced control of the aeroplane.”

We analyze how this is similar or different to the Boeing 737 MAX pitch instability issues.

………

As the AD does not apply to the in-service A320neo, the issue must not be connected to the pitch instability which comes as a natural consequence of mounting the larger neo engines on the A320 series. It can be restricted to how the A321neo version of the ELACs handles the aircraft’s controls in an excessive pitch up condition.

Since publishing the article Airbus has provided us the following information:

The issue is an A321neo landing configuration at extreme aft CG conditions and below 100ft only issue, discovered by Airbus and reported to AESA. Violent maneuvers in for instance a go-around in these conditions can cause a pitch up which the pilots can counteract using their side-sticks. No FBW nose downs or similar is commanded, it’s just the FBW doesn’t neutralize the pitch-up (like FBW using the Airbus style flight laws are supposed to do), the pilots have to do it. Airbus has assisted AESA in issuing the AD which restricts the aft CG used in operational landings until the ELAC software is updated.

Our comment: The Airbus information explains why the issue is limited to the A321neo. The A321 has a different flap configuration than the A320/A319, giving a more nose-down approach angle (a lift curve with a transposed AoA vs. lift range). It seems this difference can set the condition for the pitch-up which the FBW at this point does not compensate for. The Pilots have to do it. The FBW will take away this pitch-up in a FBW software release available 3Q2020.

………

Like the 737 MAX, the A319/320/321neos are affected by the mounting of larger engines with their larger nacelles ahead of the center of gravity, Figure 1.

Figure 1. A321 shown on top of A321neo. The larger engine nacelles are marked with a violet color. Source: Airbus and Leeham Co.

………

There are several takeaways from the above:

• As we have written in the MAX articles, pitch instabilities in certain parts of an airliner’s wide flight envelope are common.
• It comes down to how these are addressed to produce a safe aircraft. In the case of the MAX and A320, software-based control logic is used, controlling the movements of the horizontal stabilizer and elevator.
• The key is how these controls are designed, tested and implemented.
• The original MAX implementation was inacceptably badly done. It relied on a single sensor, commanded unnecessary repeated nose-down trim commands and didn’t have any global limitation on its authority.
• The Airbus version for the A321neo has a solid implementation based on adequate hardware/software redundancy and relevant limitations on its authority. But it can be improved (see our Airbus update on cause and fix).

Similar problems, and in one case they aren't killing people, because they aren't letting finance and marketing drive basic engineering decisions.