Whenever I write about the Boeing 737 Max, I can expect to hear from at least a few pilots, often from American, United, and especially Southwest Airlines, which is the biggest U.S. 737 Max customer.

Now it seems that federal criminal investigators want to hear from them as well. 

A report broke Friday that the unions representing American, United and Southwest pilots have all been hit with subpoenas from the criminal division of the US Department of Justice, requiring them to turn over 737 Max-related documents to a federal grand jury, according to Bloomberg

The unions confirmed for me Saturday that they'd been served. 

"I don't know what aspect they are investigating. They just want to know what we have on the Max. We knew it would come eventually," Jon Weaks, the Southwest union president, told Bloomberg's Mary Schlangenstein.

5 different investigations

At this point there are at least five federal investigations regarding the 737 Max. The Seattle Times compiled them all from various sources including:

  1. A criminal investigation "into the development and certification of the Boeing 737 MAX by the Federal Aviation Administration and Boeing. In this first investigation, it's being led by the U.S. Department of Justice Fraud Section, with help from the FBI and the Department of Transportation's Inspector General. 
  2. A separate administrative investigation by the Department of Transportation's Inspector General. 
  3. Hearings in front of the Department of Transportation's Inspector General. On March 27, the FAA's acting administrator, Daniel Elwell, testified; Boeing executives are supposed to testify at a later date.
  4. An FAA review panel that's supposed to look into "the certification of the automated flight-control system on the Boeing 737 MAX aircraft, as well as its design and how pilots interact with it," according to the Times.
  5. An overall congressional investigation into the "status of the Boeing 737 MAX," in front of the U.S. House Transportation and Infrastructure Committee's Transportation and Infrastructure Committee, starting May 15.

'Hundreds' more

Quick recap: this all dates back to the crash of Lion Air Flight 610 on October 29, 2018, and then the crash of Ethiopian Airlines Flight 302 on March 10, both of which were 737 Max aircraft. The crashes claimed the lives of all aboard, killing 346 people in total.

The 737 Max has been grounded worldwide since the second crash, while investigations continue regarding what went wrong -- and of how the plane was approved for flight in the first place.

Within the U.S., Southwest has reportedly made the biggest bet by far on the 737 Max, with 280 orders and 31 deliveries -- plus a plan to buy "hundreds more," according to its CEO.

American has ordered 100 and taken delivery of 24, according to the same source, while United has ordered 137 and taken delivery of 14. All three airlines' 737 Max aircraft are reportedly waiting out the grounding order in desert boneyards in the western U.S.

The 4 scariest words

It remains to be seen whether passengers will even be willing to fly on the 737 Max when it returns to service. Fully 20 percent of passengers surveyed by Barclays said they'd never fly on one, while another 23 percent said they'd wait at least a year after it returns to service before being willing to give it a try.

It's almost enough data to take President Trump's recent suggestion seriously -- although I'm still convinced that airlines that flew a rebranded 737 Max would risk their own reputations with their customers.

There's nothing suggesting any criminal wrongdoing by any pilots or the pilots' unions that I'm aware of. And a grand jury investigation is just that -- an investigation.

It doesn't mean anyone will wind up facing any criminal charges.

That said, I'm a former lawyer, and as far as I'm concerned, four of the scariest words in the English language are: "federal grand jury subpoena."

Now the pilots' unions have heard them.