Several pilots made at least five complaints about Boeing 737 Max 8 planes in the months leading up to Sunday's deadly Ethiopian Airlines crash, the second crash of a Boeing 737 Max 8 in recent months.

One captain complained about the plane's autopilot system, calling the flight manual "inadequate and almost criminally insufficient."

"The fact that this airplane requires such jury rigging to fly is a red flag," another pilot said in November.

Boeing has suffered harsh backlash over its 737 Max planes, and several countries and airlines have grounded them for inspection.

Several pilots complained about Boeing 737 Max 8 planes ahead of Sunday's deadly Ethiopian Airlines crash, the second crash of a Boeing 737 Max 8 in five months.

US records show that at least five complaints were lodged with federal authorities in recent months, with one captain even calling the flight manual "inadequate and almost criminally insufficient," according to The Dallas Morning News.

The complaints were made in the Federal Aviation Administration's incident database, which allows pilots to report issues about aviation incidents anonymously.

The complaints highlighted issues with the Max 8's autopilot system, which has been called into question after Sunday's crash and the crash of Lion Air Flight 610 in October. That incident also involved a Boeing 737 Max 8 plane.

The black box recovered from the Lion Air crash has indicated Flight 610 was repeatedly pushed into a dive position shortly after taking off from Jakarta, Indonesia. Investigators have suggested the automated system's sensors may have malfunctioned. All 189 people on board were killed in that crash.

Observers have noted similarities between the two crashes, with Ethiopian Airlines CEO Tewolde GebreMariam telling CNN the similarities were "substantial."

"He was having difficulties with the flight control of the airplane, so he asked to return back to base," GebreMariam said.

Complaints focused on the autopilot system

A Boeing 737 Max outside the hangar at the Boeing plant in Renton, Washington. Reuters

In one complaint, a commercial pilot described issues occurring during takeoff. As the autopilot was engaged, the aircraft's nose suddenly pitched down, setting off the plane's alarm system, which sounded "Don't sink, don't sink!" according to Politico. The situation was remedied only after autopilot was turned off, it added.

Another pilot who flies the Max 8 complained in November that it was "unconscionable" that pilots were allowed to continue to fly the planes without more training or disclosure on how the Max 8's system differed from previous models, Politico said.

"The fact that this airplane requires such jury rigging to fly is a red flag," the pilot said in the report. "Now we know the systems employed are error prone — even if the pilots aren't sure what those systems are, what redundancies are in place, and failure modes."

Read more: Germany, the UK, China, and other countries have grounded the Boeing 737 Max 8 after its 2 deadly crashes — here's who's taken action so far

In an October report, a pilot complained the Max autothrottle system did not work properly. The problem was rectified after the pilot adjusted the thrust manually and continued to climb.

"Shortly afterwards I heard about the (other carrier) accident and am wondering if any other crews have experienced similar incidents with the autothrottle system on the Max?" the pilot wrote in the report.

Boeing faces backlash

Boeing has faced backlash over its Max planes as investigators look into what happened to both doomed flights.

Countries and airlines around the world have begun grounding Boeing 737 Max planes for inspection.

On Tuesday, Boeing's market value had plunged by $40 billion from its 2019 peak. Shares have slumped about 15% since topping out on March 1.

Read more: 'If it's Boeing, I'm not going': People are freaking out about flying on the plane model that has now crashed twice in 5 months

Boeing has announced that all Max planes will receive updated flight-control software in the coming weeks, though the company has not indicated whether it will make physical changes to the aircraft, which has been in service since last spring.

"At the heart of the controversy surrounding the 737 Max is MCAS, the Maneuvering Characteristics Augmentation System," the Business Insider transportation correspondent Benjamin Zhang wrote.

"To fit the Max's larger, more fuel-efficient engines, Boeing had to redesign the way it mounts engines on the 737. This change disrupted the plane's center of gravity and caused the Max to have a tendency to tip its nose upward during flight, increasing the likelihood of a stall," Zhang said.

"MCAS is designed to automatically counteract that tendency and point the nose of the plane downward."