Exclusive: AAIB silent on stabilizer motor failure on flight hours before Air India 171 crash

In Part 1, 2 and 3 of The Federal investigation into the Air India 171 crash, we looked at how core network degradation caused multi-component failure, and how the airline has been speeding up extensive D-check of its Dreamliners.

In part 4, we will look into how the Aircraft Accident Investigation Bureau’s (AAIB) report mentions a stabilizer trim sensor issue on the previous flight (AI 423 Delhi-Ahmedabad) of the same plane, but does not mention a bigger problem with the stabilizer motor unit.

Hours before Air India 171 plunged to the ground — killing 260 lives in one of India’s deadliest aviation disasters — the same aircraft (VT-ANB) experienced a hard landing at Ahmedabad airport on its previous leg due to some faults, The Federal has learnt from maintenance records.

And this pattern of faults appearing on the said aircraft was seen throughout the day on June 12, 2025 — first, during the landing of the morning flight (AI 423) and, later, before take-off on flight AI 171 that crashed. At 1.23 pm, 15 minutes before take-off, Air India 171 started flashing electrical faults, as we reported in part 3.

So let’s rewind to a few hours before that tragic afternoon, when the earlier flight (AI 423) experienced a hard touchdown, triggered by faulty stabilizer-trim sensor inputs from the tail of the aircraft. These inputs are needed by the flight-control computers so that they can make continuous adjustments to the aircraft’s control surfaces and pitch, and keep it stable. When the flight computers don’t get these stabilizer trim inputs, a plane’s landings and take-offs can become difficult.

The Federal now has exclusive evidence that on that fateful day, the Boeing 787 (VT-ANB) not only had a sensor issue in the tail, but its entire right stabilizer motor was faulting and had to be replaced.

The hard descent of AI-423, which in its next leg became AI-171 (Ahmedabad–London) did raise an alarm. Once the plane taxied to the gate, the pilots made an entry in the pilot handbook about the stabilizer trim issue.

The AAIB report refers to the stabilizer issue, saying: “The crew of the previous flight (AI 423) had made Pilot Defect Report (PDR) entry for status message ‘STAB POS XDCR’ in the Tech Log.” The sensor, also known as the Stabilizer Position Transducer or SPT, informs the pilot and systems about the position of the aircraft.

A passenger, who wished to remain anonymous, said on flight AI 423, there was an announcement some 10 to 15 minutes before landing from the pilot, Capt. Hardeep Deol, that they should expect “some turbulence”.

The deeper fault AAIB didn’t mention

But the AAIB report fails to mention the problem was more severe. The Federal has gained access to the Aircraft Health Management (AHM) report that was sent to Boeing at 9.48 am IST (for Flight AI 423) that shows not only was the sensor showing stabilizer position malfunctioning but also the electronic control box that drives one of the tail-trim motors called the horizontal stabilizer electric motor control unit (EMCU). Both were replaced and the plane released for flight.

On this incident, the AAIB report says, “The troubleshooting was carried out as per FIM by Air India’s on-duty AME, and the aircraft was released for flight at 06:40 UTC (12:10 PM IST).” What this translates to is that the aircraft maintenance engineer (AME) did the troubleshooting as per Boeing’s fault isolation manual (FIM).

A bad motor or a bad power domain?

On the Boeing 787, the horizontal stabilizer trim isn’t moved by cables or hydraulics; it’s driven by EMCUs, powered from the aircraft’s low-voltage DC distribution network.

Now, before the stabilizer motor went bust on June 12, its sibling in the power domain, the fire inerter or nitrogen generation system (NGS), had already faulted two days earlier. On June 10, 2025, the fire inerter — which helps prevent fuel tank fires — was marked a high-risk active fault or CAT A MEL.

The NGS works by continuously flooding the tail fuel tank’s ullage (the empty space above the fuel) with nitrogen-rich air, displacing oxygen and thereby preventing the build-up of flammable vapours.

In aviation parlance, active faults are called Minimum Equipment List (MEL), which is a list of specific equipment or systems that can be inoperative but the aircraft can still fly if certain conditions or restrictions are followed. These are categorised from CAT A to CAT D — A being a fault with the highest risk that must be fixed within 24 hours to D being the lowest that could be attended to between 120 and 160 days.

So, how is the stabilizer motor a sibling to the fire inerter? Both share their electrical ancestry back to the same source — the engine-mounted main generator or the variable frequency starter generator (VFSG), which feeds the 235-volt AC main buses.

Power paths:

R2 235 V AC main bus → ATRU R2 → ±270 V HVDC → CMSC R2 → NGS/ CAC-R2 / Hyd L EMP

L2/R2 235 V AC main bus → PCS (115 V/±130 V/28 V) → right stabilizer EMCU (115 V/±130 V) → right stabilizer sensors (28 V)

The alphanumeric characters indicate the power lines or paths that control one of the independent electrical channels designated L2/R2 (Left or Right) to ensure continued operations in case of a single channel failure. In this case, the primary power path for the right stabilizer was line L2.

Now, L2 is also the line used by a right hydraulic system interface (Hyd R) that faulted a few hours later. It is a network of components that connect the aircraft’s three hydraulic systems that power flight controls, landing gear, and brakes.

So, seeing power surges in that line, the stabilizer trim would then use its secondary power path R2, which would make it vulnerable to whatever impacted the fire inerter. And that tracks with the record: engineers had to take the fire inerter (NGS) offline two days before the crash, and the right stabilizer motor and sensor had to be replaced on the day of the crash.

Core network issue: CAT C MEL

On the Boeing 787, the fire-inerter and stabilizer trim motor are not isolated; they share both power and data with the aircraft’s core network—a digital-electrical nervous system that manages most on-board systems and was listed as a “medium-risk” active fault or Category C MEL on June 9, three days before the crash, according to the AAIB preliminary report.

The horrific Ahmedabad crash has shaken Air India engineers, who now say following Boeing’s FIM to a tee would not have fixed the deeper problem: the power architecture feeding those units. They feel a review of the certification process for faults or MEL categorisation might help improve flight safety.

One network feeds everything — from core computers to cabin climate

Boeing literature shows the 787’s core network connects about 22 flight-critical systems such as the computers, servers, engines, data recorders, etc., and 28 more mundane, non-critical features like the in-flight entertainment screens, lavatory lights, cabin climate, and crew communication panels.

So, if the core network was connected to flight-critical systems, why was it marked medium-risk?

This is where it is important to note that was not a call taken by Air India or Air India Engineering Services Ltd (AIESL) engineers. They went by the risk as classified by Boeing and certified by regulators like the US Federal Aviation Administration (FAA) and Directorate General of Civil Aviation (DGCA India), which meant that the engineers were required to treat this degradation as medium-risk or CAT C MEL.

When core network malfunctions, engines can pay the price

Where it gets dangerous is that the core network is the system that feeds, validates, and arbitrates signals for the engine computers or (Full Authority Digital Engine Control) FADECs. So, according to aviation experts, when this network is degrading, its corruption won’t be localised — it can flow into FADEC logic to command thrust and fuel cut-off.

And veteran safety investigators say these kinds of faults, when ignored, have the potential to show up drastically in-flight. Ed Pierson, executive director, Foundation for Aviation Safety, says, “Intermittent system failures could be indicators there was something wrong with the electrical system on the plane.”

The former Boeing senior manager added, “For instance, even in the 737 MAX disasters, there were electrical system failures occurring nearly a month before the crashes.”

No fire inerter, no protection at impact

And was the faulty fire inerter or NGS just an indicator of an existing electrical problem on the plane? No. If the nitrogen-generation system — the unit that’s supposed to inert the fuel tanks by replacing oxygen with nitrogen — was inoperative, then the fuel tanks would have been sitting “live”, with oxygen-rich vapour that could ignite on impact.

That means, there was nothing to slow or suppress an immediate fuel fire when the aircraft hit. If the inerting system had been doing its job, it’s possible the initial fireball would have been smaller and more survivable — and AI 171 might have had more than one survivor, said aviation experts.

Neither Boeing, nor Air India, DGCA India, AAIB, or other regulatory agencies like the EASA have responded to The Federal’s request for comment.

(Disclaimer: The AAIB has not yet released its final report on the AI-171 crash. All the technical scenarios presented here are based on preliminary information and remain hypotheses)