November 27, 2020, ©. Leeham News: After the lifting on the grounding order by the FAA, ANAC (Brazils regulator) followed in the week, and EASA issued its plans for public comment.
What are the differences in the ungrounding conditions, and what are the reasons for any differences?
ANAC, Brazil’s regulator, which has the competence to certify airliners through decades of work with Embraer, lifted its grounding order for the 737 MAX Wednesday. ANAC based it on FAA’s conditions with no changes, but in comments around its lifting on the grounding, it refers to ongoing work on further improvements to the MAX. The one Brasilian airline that operates the MAX, GOL, can immediately update its 737 MAXs and retrain its crews.
One can only speculate what ANAC means with the “further improvements,” but these likely include the third synthetic AoA sensor that EASA wants and some further clean-up of the warnings in the 737 cockpit. I write “737 cockpit” and not 737 MAX cockpit as a large factor in operating the 737 MAX is its use with a 737 NG fleet. In such a case, the airline and the certification authorities want to minimize any differences in the types’ behaviors.
EASA, the EU regulator, Tuesday issued a “Notification of a Proposal to issue an Airworthiness Directive.” This is its candidate for the un-grounding Airworthiness Directive (AD), published for public comment. The comment time ends 22nd of December, then the comments shall be reviewed by EASA. We can expect the AD by mid-January.
The AD proposal is based on the FAA ungrounding directive, but It adds a couple of conditions and restrictions.
The simplest condition is the requirement for the top part of the stick shaker system’s circuit breakers to change from black to red color. It’s to make them easier to identify should a crew decide to silence an erroneous stick shaker.
The circuit breakers in the 737 are placed on the back wall of the cockpit, Figure 1. Pulling the breaker requires a crew member to un-buckle and at least lean over to pull the circuit breaker (the Captain’s Stick Shaker breaker is behind the First Officer and vice versa). Is this safe or not?
Well, the crew is allowed to stand up and go to the WC, so leaning over or standing up to pull a circuit breaker should be no different. A decision to pull the breaker will only be taken in a situation of full control of the aircraft. It’s difficult to identify the correct Circuit Breakers (and pulling the wrong one is not good) so making the Stick Shaker caps red help.
FAA says there is no need to silence a rough stick shaker while EASA (and Transport Canada we presume) says it improves the recognition of the other side Stick shaker should the aircraft run into a real high AoA situation. As there is no urgency to silence the asymmetric rotating counterweight attached to the Yoke column, pulling the breaker for an erroneous Stick Shaker should be OK.
The other condition has its root in the disconnection of Speed Trim, MCAS, Autopilot, and Flight Directors should the two Angle of Attack systems disagree. EASA will temporarily revoke the 737 MAX certification for Required Navigation Performance – Authorization Required (RNP AR) approaches.
Such approaches are demanding as they are often curved approaches in valleys to difficult to access airports. Look at this famous Queenstown, New Zeeland RNP-AR approach, and you see what I mean. Imagine sticking your nose down there when it’s all covered in clouds.
Should the AoA monitor trip, Speed Trim, MCAS, and more importantly, Autopilot and Flight Directors disconnect, it puts a crew in a very tight spot as the difficulty of such approaches are high (they require special crew training and certification). You need all the tools you have in such approaches and don’t want a sudden disconnect of the Autopilot and Flight Directors combined with Speed Trim warning, followed by AOA, IAS and ALT DISAGREE.
The revoke of the RPN AR approach certification is temporary. One can guess it will be allowed again once a synthetic third AoA sensor is introduced to the MAX. It creates a voting “two versus one” situation when one of the sensors presents suspicious values. It would then result in an AOA DISAGREE warning, but the Autopilot and Flight directors would stay on and IAS and ALT would still get the required AoA corrections. The AOA DISAGREE is then an indication for required maintenance rather than a major system hiccup.
The EASA decision makes sense, and one would wonder what the situation in the FAA jurisdiction is? Are there no operators that fly the demanding RNP AR approaches? Or has the FAA another view on what is OK?
The restriction makes EASA’s argument for a third sensor understandable. For regular operations, the two AoA sensor solution is OK, but something better is required for demanding operations. The revoking of the RPN-AR certification is the way for EASA to allow the 737 MAX back to service with two sensors while Boeing gets time to do the software changes to create a third, synthetic sensor. This then enables more advanced approaches.
Is the non-allowance of RNP AR approaches a significant restriction for EU operators? To my knowledge, no.
RNP AR approaches are rare as they require specially equipped aircraft and require the airlines and their pilots to be specifically trained and certified for shooting this type of procedure.
They are the last resort when nature prohibits standard approaches like ILS or in the future GLS (GPS approach similar to ILS CAT II and III, with local correction of the GPS signals).
Both ANAC and EASA require training along the lines of the FAA. EASA point’s out training preparations can start now, no need to wait for the AD to come into effect after the comment period. The simulator requirement will occupy the 50 MAX simulators that are spread over the world. EASA warns that it will take time for all Pilots to complete the 737 MAX syllabus.