Shares in troubled airplane maker Boeing (BA) slipped more than 4% on Tuesday as a slowdown in production following a series of highly publicized safety incidents and manufacturing glitches continue to weigh on the company's stock.

Recent announcements from leading airlines have put the spotlight on Boeing’s stalled plane deliveries. Low-cost carrier Southwest Airlines (LUV) said on Tuesday that it plans to slash flight capacity and review its earnings guidance due to lower-than-expected deliveries from the plane manufacturer. 

Perhaps more troubling for Boeing shareholders, United Airlines (UAL) CEO Scott Kirby told analysts at the JP Morgan Industrials Conference on Tuesday that the carrier is considering ordering rival Airbus (EADSY) A321neos to replace previously ordered Boeing 737 Max 10s due to certification uncertainty related to production and quality issues.

“As much as I would like those deliveries, this is not a 12-month issue,” Kirby said in relation to the airline’s Boeing orders. “This is a two-decade issue,” he added.

The airline had ordered 151 Max 10s initially intended for delivery over the next two years, but has recently removed those jets from its fleet plan due to Boeing’s production challenges. Currently, United has six A321neos in service and 174 of the jets on order.

Boeing has wound back production of its top selling 737 MAX planes in recent months amid heighted scrutiny from the Federal Aviation Administration (FAA) after a series of safety incidents and production glitches, most notably a door plug blowing out of a MAX 737 operated by Alaska Airlines (ALK) mid-flight in January.

The company said it delivered 17 narrow-body Max jets last month, eight less than it did in January and about half the amount it handed over to customers in November and December.

Boeing shares continue to nosedive after breaking below a long-term uptrend line stretching back to late September 2022. Moreover, the 50-day moving average crossed below the 200-day moving average last week to form an ominous death cross—a chart pattern that often marks the start of a new downtrend. It’s also worth noting that trading volume has increased in recent days as the share price has fallen, indicating active participation in the sell-off.

Looking ahead, investors should keep an eye on a key 18-month horizontal line around $174 that may provide a near-term floor of support.

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As of the date this article was written, the author does not own any of the above securities.