As if air travel weren’t stressful enough in this era of long lines, cramped seating, and overstuffed planes, computer outages have added the prospect of delayed or cancelled flights to the list of flyers’ worries. It’s a legitimate concern, and a growing one.

On July 20, a computer meltdown at Southwest resulted in more than 2,000 delayed or cancelled flights, inconveniencing tens of thousands of travelers. Then, in early August, Delta’s computers crashed, resulting in thousands more flight delays and cancellations over several days. And in September, British Airways’ check-in software malfunctioned, causing delays at airports worldwide, waits of up to five hours, and an untold number of missed connections.

This weekend, United became the latest major airline to subject its customers to a computer-related service disruption, as hundreds of flights were grounded due to what the airline described as an “IT issue.” Tens of thousands of flyers were inconvenienced.

The frequency and severity of the meltdowns—four incidents within six months, affecting hundreds of thousands of travelers—has raised concerns in Congress, led by Senators Richard Blumenthal (D-Conn.) and Edward J. Markey (D-Mass.). In September 2016, they sent a letter to the 13 largest airlines, criticizing them for their refusal to invest in the systems critical for operating safe, reliable air transport, and questioning the adequacy of the their policies and procedures for handling consumers affected by service disruptions.

And following the recent United incident, they sent a letter to United chief Oscar Munoz, requesting a full accounting of the latest failure and information on the company’s plan to prevent further lapses:

In a world where consumers can find, purchase, and check in for flights from their smartphones, IT failures should not be grounding entire airline fleets. Now that three of the four largest air carriers have recently experienced significant disruptions due to IT failures, it is time for airlines to update their IT systems for the jet age.

The Senators also took aim at the airline’s policies for handling travelers affected by the disruptions:

Further, in the event of irregular operations caused by air carriers, airlines should assist passengers, including by rebooking passengers on another airline or on a different mode of transportation, without charging additional fees.

The letter closes by posing seven pointed questions regarding the past, present, and future state of United’s IT operations, and asks Munoz to respond by February 6. The Senators are awaiting his reply, as are United’s customers.

Reader Reality Check

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After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.