The swift return of Boeing’s 737 Max aircraft to the skies was put further in doubt this week with airlines signalling that they do not hope to operate the plane any time soon.

With more than four months already elapsed since the plane was grounded by regulators, Southwest and American, two of the jet’s main US operators, followed United Airlines in saying they would be taking the Max out of their schedules until November.

Ryanair, Europe’s biggest short-haul carrier, also announced it would have to curb expansion plans pinned on the arrival of its 737 Max orders and that some airport bases would have to shut as a result.

Making the announcement on Tuesday, its chief executive, Michael O’Leary, said he remained committed to the plane: “We’ve described them as gamechangers – and they remain gamechangers.”

But O’Leary admitted even its biggest customers – Ryanair has ordered 135 models – have little visibility on its immediate future: “We’re still operating in the realms of considerable uncertainty ... there are no guarantees.”

According to one report, the 737 Max might not return until next year as Boeing searches to fix the design flaws behind two deadly crashes. Boeing’s top-selling plane, with around 5,000 on order worldwide, was pulled from service after an Ethiopian Airlines crash outside Addis Ababa in March that killed all 157 people on board. It was the 737 Max’s second disaster in five months, after 189 people were killed on a Lion Air flight in Indonesia in October.

Elsewhere this week, it became clear that restoring trust among passengers could take longer than fixing the plane. In Washington, relatives of passengers who died in the Ethiopian disaster told Congressional hearings that Boeing had focused on profits “at the expense of human life”. In a blistering attack on the manufacturer and the US regulator, the Federal Aviation Authority (FAA), Paul Njoroge, who lost five family members including his wife and three children in the crash, warned that, without change, “another plane will dive to the ground, killing me, you”.

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Boeing released an initial $50m (£40m) in compensation for the bereaved on Wednesday, as part of a $100m fund it has set aside for victims’ families. The aerospace group also announced on Thursday it would take a $4.9bn charge reflecting the cost of compensation to airlines for schedule disruptions and a slowdown in production.

Weeks of silence on the software issues at the centre of the crash investigations will worry airlines. Investigators are focusing on the 737 Max’s new anti-stall software, the Manoeuvring Characteristics Augmentation System (MCAS), which was designed to keep the aircraft stable in flight. In both cases they suspect that the MCAS software – which automatically pushes the aircraft’s nose down to guard against a loss of lift – was activated by erroneous data from a single sensor. This forced the aircraft into dives that the pilots, ultimately, could not stop.

In the wake of the Lion Air crash in October, Boeing started work on a fix. But in late June, when Boeing and the FAA appeared to be reaching a conclusion, they instead revealed a second flaw had been discovered, beyond MCAS. It was another “potential source of uncommanded stabiliser motion”. Or, another way the plane’s computers might battle unwitting pilots in the cockpit, should it think its nose would be better pointing down.

According to the global airline body, Iata, restoring the plane to service would take another five weeks from regulatory approval by the FAA. Boeing hopes to present its full bid for recertification in September, but the US carriers’ rescheduling moves show they do not envisage an immediate sign-off.

Approval is likely to be checked by non-US regulators, virtually all of whom grounded the plane before the FAA. That schism caused profound concern in the aviation industry and the FAA is unlikely to go it alone now. Iata, which hosted a summit to urge regulators to align, warned that aviation relied on “mutual recognition, trust, and reciprocity among safety regulators ... Restoring public confidence demands it.”

Retraining 737 Max pilots is another moot question. The Max was sold as an upgrade to the existing, bestselling 737, rather than as a new type: removing the expense of extra training, beyond a short iPad session. Many pilots doubt this was enough – redoubled since the Ethiopian crash, where flight crew had followed Boeing’s instructions and reminders after the Lion Air disaster. Should Boeing and the FAA decide it necessary to refer Max pilots to simulators – even as a confidence-restoring measure – further delays would ensue. This would hit early adopters such as Norwegian, who are deep in the red and desperate for the economies the Max would bring.

Some have more radical suggestions. Ralph Nader, the consumer advocate and former US presidential candidate, whose grand-niece Samya Stumo died in the Ethiopia crash, has demanded a full, transparent review of the aircraft before recertification. He says the review should be overseen by a public panel including victims’ families, pilots, cabin crew and mechanics, as well as the FAA experts.

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Some think the plane may never fly again – at least as the Max. Donald Trump has already tweeted a suggestion to change the name. Boeing executives at June’s Paris Air Show appeared to consider a rebrand, but the idea was slapped down by the boss. Chief executive Dennis Muilenburg said he did not “see the need”, adding: “This is not a marketing or branding exercise. Certainly it impacts the public view, but the most important thing is safety.”

Yet a Ryanair Max 200 emerging from the Boeing factory in Renton, Washington, this week had the designation “737-8200” on the nose cone, instead of the 737 Max seen on other planes. Who oversaw the paint job remains a mystery. Boeing indicated it was a question for Ryanair, but O’Leary later said it was “nothing to do with us ... We just put the names and a big Ryanair on them.”

O’Leary does not anticipate customer concerns when the planes are certified and delivered, pointing to the example of the 787 Dreamliner. The 787 was temporarily grounded due to safety concerns in 2013 but is now popular with the public, he said. Likewise, the Max “will be warmly welcomed by passengers who fly on them”.

O’Leary added: “The [Max 737] operating costs are phenomenal; the fuel consumption, and additional revenue opportunity, while still operating with only four cabin crew,. If Boeing could deliver them tomorrow, we’d take them.”