Allegiant Air is a small, ultra-low-cost carrier based in Las Vegas, that happens to be one of the country's most profitable airlines. But, according to federal aviation records and interviews with pilots, mechanics and industry experts, it may also be the most dangerous.

The airline flew 12 million passengers last year on its 99 planes to 120 destinations from California to Florida. But it has had persistent problems since at least the summer of 2015 when it experienced a rash of mid-air breakdowns, including five on a single day. It was not a fluke.

Public documents show an alarming number of aborted takeoffs, cabin pressure loss, emergency descents and unscheduled landings. Yet for the most part, Allegiant's difficulties have managed to stay under the radar of the flying public.

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It's entirely possible that you have never heard of Allegiant or flown on one of its planes. But if you shop for the cheapest ticket, live near cities like Pittsburgh or Cincinnati that are underserved by major airlines or you rely on regional airports, then you probably recognize the company's colors and logo.

Allegiant has some of the lowest fares, the least frills and the oldest fleet in the business. Right now, nearly 30 percent of its planes are antiquated, gas-guzzling McDonnell-Douglas MD-80s, almost all of them purchased second-hand from foreign airlines. It also has more than its share of angry, traumatized passengers willing to share their experiences.

Dan Mannheim: People are screaming. The stewardess are running up and down the aisles.

Chris: And then the smoke started pouring in out of all the vents you know, started filling the cabin up with smoke.

Shanyl: All I kept thinking was, "Thank God we're on the ground."

"I have encouraged my family, my friends and myself not to fly on Allegiant."

For the past seven months, we have been scrutinizing 'service difficulty reports' filed by Allegiant with the FAA. They are official, self-reported records of problems experienced by their aircraft. What we found raised some disturbing questions about the performance of their fleet. Between January 1st, 2016 and the end of last October, we found more than 100 serious mechanical incidents, including mid-air engine failures, smoke and fumes in the cabin, rapid descents, flight control malfunctions, hydraulic leaks and aborted takeoffs.

John Goglia: Something significant is going on and it should be addressed

We shared the reports with John Goglia who has more than 40 years of experience in the aviation industry, including nine years as a presidential appointee to the National Transportation Safety Board. Now retired, Goglia remains a respected figure in the aviation industry and occasionally testifies as an expert witness on safety issues.

John Goglia: There's another one, engine fire.

We wanted to know what he thought of Allegiant's 60 unscheduled landings and 46 in-flight emergencies.

Steve Kroft: I mean, is that common for an airline of this size?

John Goglia: Very, very high for an airline of this size. I hate to make comparisons-- but we've seen that before in airlines that are no longer with us that had experienced a number of accidents and killed a bunch of people. I don't wanna repeat that. So I try to push on Allegiant to-- to-- clean up their operation.

Steve Kroft: What do those reports say about Allegiant?

John Goglia: Well, just the service difficulty reports say that-- somebody's not paying attention.

Steve Kroft: You're a former member of the NTSB. Would you fly on an Allegiant plane?

John Goglia: I have encouraged my family, my friends and myself not to fly on Allegiant.

We wanted to ask Allegiant and its CEO, Maurice Gallagher about all of this, what they gave us instead was a brief statement from their vice president of operations which says, in part:

"All of us at Allegiant are proud of our strong safety record, as noted in the most current, comprehensive FAA audit. Safety is at the forefront of our minds and the core of our operations."

But John Goglia and other aviation experts we talked to aren't so sure. They believe Allegiant's problems come from the confluence of its aggressive business model and a safety culture they find to be lagging. The business strategy which has produced 60 straight quarters of profits, occasionally with margins approaching 30 percent, requires the airline to keep costs down and "push the metal" -- keep the planes flying as often as possible. But Allegiant's aged fleet of MD-80s, which it is phasing out and is responsible for most of its problems, require a lot of maintenance and reliable parts are hard to come by.

Steve Kroft: Is there anything that separates the maintenance systems at Allegiant from the ones at the larger carriers?

John Goglia: Well, the first and most obvious piece is the lack of infrastructure. They don't have the number of mechanics. And we've seen some problems with the contractors that they've used. We're seeing problems that require-- feet on the ground, people looking at the airplanes when they're being worked on so that these problems are caught during maintenance and not caught by the f-- crew as-- as a surprise and emergency.

We found numerous planes with the same recurring issues and others returned to service before they were ready. Like Allegiant Flight 533 last July, which was delayed in Cincinnati on its flight to Las Vegas.

Mercedes Weller and Dan Mannheim, who says he paid $80 for his roundtrip ticket to Vegas, remember the pilot's announcement as they pushed away from the gate three hours behind schedule.

Mercedes Weller: He came on and he said, "The mechanics have been working on this right engine. We apologize for that. We'll get you up in the air as soon as possible." As we started taxiing, everything was going okay. And then it's, like, as soon as the wheels came up, the engine blew.

Passenger Video: Here we go, we're taking off. Say blastoff! Oh [expletive].

Mercedes Weller: The force of it was so hard that it-- it popped open the cockpit doors. And there was smoke in the cabin and fire coming out of that engine. And I just remember thinking that I would never see my daughter again.

Weller and Mannheim said the plane had to circle at a low altitude on one engine for about 25 minutes while the airport ground crews cleared debris from the runway for an emergency landing.

Dan Mannheim: Everyone turned their phones back on. And I called my family. And-- I pretty much tellin' 'em goodbyes.

Steve Kroft: You thought this was it?

Mercedes Weller: I text my husband. And I said "If something happens, just know that I've been very happy. And I love you."

The plane eventually landed safely back in Cincinnati. For their trouble, Allegiant offered to re-book Mercedes and Dan the next day and gave everyone a $150 voucher.

But that was not the only problem Allegiant's small fleet encountered last July. There were nine other Allegiant planes that also had to make unscheduled landings during that month. Four of those planes had engine problems, two reported fumes in the cabin, four had instrument or flight control problems. And that's not all.

John Goglia, who spent nine years as a presidential appointee to the National Transportation Safety Board CBS News

Steve Kroft: Over the course of one weekend in July, Allegiant canceled or rescheduled 11 separate flights leaving Las Vegas, all for mechanical issues.

John Goglia: You think that's a big red flag? Yeah--

Steve Kroft: Something's wrong

John Goglia: Something's wrong.

Among the most concerned are the people that have to fly the planes. Daniel Wells, a captain for Atlas Air, with 30 years experience, is president of Teamsters Union 1224 which represents pilots from Allegiant and nine other airlines. We wanted to know how unusual it was for a small airline with 99 planes to have 25 engine failures or malfunctions in less than two years.

Daniel Wells: Well, I-- I don't have all the data in front of me to compare with other airlines. But I can say that those are extraordinarily high numbers.

Steve Kroft: Outside the norm?

Daniel Wells: Outside the norm for sure. If I come into a career as an airline pilot now, I will go my entire career, maybe 30 years, and never have an engine failure, ever.

Steve Kroft: What are Allegiant pilots telling you about their airline?

Daniel Wells: What I hear from hundreds of conversations with Allegiant pilots, is the management of Allegiant seems to denigrate the pursuit of safety.

Steve Kroft: Why are we unable to talk to any Allegiant pilots?

Daniel Wells: Well-- (LAUGH) I think that says volumes about the company. I would love to put up some of the-- Allegiant pilots. But they can't. And they can't because they know that they would be terminated. At the very least, disciplined. And that's just for speaking up about concerns. So I have to speak on their behalf.

Captain Wells says they have every reason to fear retaliation considering what happened to one of Allegiant's pilots three summers ago.

On June 8, 2015, this Allegiant MD-80 jet with 141 passengers aboard had just left St. Petersburg, Florida for Hagerstown, Maryland when a flight attendant informed the pilot that there was smoke in the cabin.

ATC RADIO TRANSMISSION:

Allegiant 864: This is Allegiant 864, declaring an emergency at this time.

Concerned about a fire, Captain Jason Kinzer and his co-pilot made a quick decision.

Allegiant 864: We need to return to St. Pete's. The flight attendants are reporting smoke in the cabin.

Tower: Allegiant 864. Roger. Cleared back to St. Pete airport.

Captain Kinzer landed the plane and was met at the end of the runway by fire and rescue trucks that confirmed his concerns.

ATC RADIO TRANSMISSION:

Rescue Fire: I'm showing some smoke on your number one engine.

Allegiant 864: Verify…you're showing smoke on the number one engine?

Rescue Fire: That's affirmative.

Allegiant 864: Uh, Tower ground. Allegiant 864 we're going to be evacuating.

Rescue Fire: 864, roger.

Unidentified Voice: 864, hold off on your evacuation, please.

Allegiant 864: Who said to hold off?

It would take another 22 seconds and prompting from ground control before that voice identified himself as 'Rescue Fire 2'. But, with the clock ticking and confusing radio chatter, Kinzer did what he had been trained to do: he deployed the emergency chutes and evacuated the aircraft, with eight people sustaining mostly minor injuries.

The incident drew unwanted attention to Allegiant but nothing compared to what followed six weeks later when the airline abruptly fired Kinzer for his actions.

Daniel Wells, a captain for Atlas Air, with 30 years experience, and president of Teamsters Union 1224 which represents pilots from Allegiant CBS News

Daniel Wells: I haven't spoken to a single captain, both Allegiant and otherwise that knows the details, that didn't say, "He absolutely did the right thing. And if I was there, I woulda done exactly the same thing."

Loretta Alkalay: I've never ever heard of an airline firing a pilot for an emergency evacuation.

Loretta Alkalay has a lot of experience in things like this. She spent 30 years at the FAA prosecuting enforcement cases in the northeast region. She was particularly annoyed by Allegiant's letter of termination that blamed Kinzer for what it called an "evacuation that was entirely unwarranted" and for not "striving to preserve the company's assets".

Loretta Alkalay: Yeah, it's really-- it's really outrageous. And-- and that's where the FAA should have stepped in to look at the safety culture because the message to all the other Allegiant pilots is: don't ever have an emergency evacuation if you don't see flames in the cockpit. I mean, that's-- what other message could you get?

Steve Kroft: Did the FAA investigate?

Loretta Alkalay: I have been told that they never questioned the-- the pilot.

We decided to question the FAA.

Steve Kroft: As far as the FAA is concerned, what's more important: the safety of the passengers or the assets of the airliner?

John Duncan: Certainly the safety of the passengers is what's always important.

John Duncan is the executive director of flight standards at the Federal Aviation Administration. He oversees the operations, maintenance and airworthiness of all U.S. carriers.

Steve Kroft: That doesn't seem to be the values of Allegiant as expressed in this letter of termination. You've seen it, right?

John Duncan: I've seen the letter, yes. But just that letter, and-- and nothing else, so I don't know the rest of the story in that regard.

Steve Kroft: So nobody asked you to conduct an investigation so you haven't?

John Duncan: To my knowledge we haven't been asked to investigate, through a whistleblower complaint, the termination of the captain.

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Steve Kroft: What do you think the FAA should have done?

Loretta Alkalay: Well, they should have been all over that. I mean, it's outrageous. What kind of safety culture would allow that to happen. I mean, it's just so inimical to safety.

Captain Jason Kinzer filed a wrongful termination lawsuit against Allegiant, which is scheduled to be tried next month. As for the FAA, which is charged with enforcing airline safety in the U.S., it has not brought a single enforcement action against Allegiant in nearly three years. How can that be?

Allegiant Air is an ultra-low-cost carrier primarily known for its rock-bottom fares and its high profit margins. But what really sets it apart from the competition is that its planes have been nearly three and a half times more likely to have serious in-flight mechanical failures than other U.S. Airlines. That figure comes from a seven-month review we conducted of safety records on file with the Federal Aviation Administration. What's equally surprising to us is what some have called the FAA's passive approach to correcting Allegiant's difficulties.

It has to do with a change of policy. Over the last three years, the FAA has switched its priorities from actively enforcing safety rules with fines, warning letters and sanctions-which become part of the public record-to working quietly with the airlines behind the scenes to fix the problems. It may well be what's allowed Allegiant to fly under the radar.

But Loretta Alkalay, who spent 30 years as an FAA lawyer, says it does not absolve the agency of its responsibility to ensure airline safety.

Steve Kroft: The FAA's job is to enforce airline safety, isn't it? Isn't that part of its job?

Loretta Alkalay: Yes. When the FAA knows that an airline has a problem, or there are sufficient red flags, it is supposed to step in and protect the public because airlines have to operate, by statute, to the highest level of safety.

Steve Kroft: And there are red flags here, in the case of Allegiant.

Loretta Alkalay: Yes, there's definitely red flags.

But you wouldn't know it talking with the FAA. We asked John Duncan, the executive director of flight standards, to comment on the more than one hundred serious mechanical incidents we found at Allegiant in a 22-month period ending last October.

Steve Kroft: Multiple engine failures, aborted takeoffs, cabin pressure loss. That's 100 serious incidents.

John Duncan: And we take those very seriously. And we look at each one of them. We look for the root cause, and-- and then we address that root cause, and assure that a fix is put in place to make sure that that problem is resolved and doesn't reoccur.

Steve Kroft: And you're satisfied that all of the problems with Allegiant have been fixed?

John Duncan: We're satisfied that-- that we are-- we are taking the appropriate actions with regard to Allegiant and every other carrier that we work with to make sure that those problems have been-- have been appropriately dealt with.

Looking at the FAA's records, you would have to conclude that that is a very optimistic assessment. Go back to August 17, 2015 -- around the time the FAA switched priorities from enforcement to compliance -- and you can see the differences in their approach. Allegiant Flight 436 was leaving Las Vegas full of passengers when it nearly crashed on takeoff. Barreling down the runway, the pilot had trouble controlling the plane. Running out of asphalt, he made a last-second decision to abort, traveling at 120 knots per hour, barely avoiding disaster.

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John Goglia: Something inside him said, "I'm not putting this in the air." And thank God he didn't, because that was-- gonna result in a bad outcome.

The problem turned out to be a missing cotter pin that holds together essential components necessary for the pilot to fly the plane. John Goglia is a former member of the National Transportation Safety Board.

John Goglia: I mean, this is a critical flight control. So this isn't-- isn't fixing a coffee maker. This is fixing a critical flight component and obviously, that wasn't done adequately.

According to the detailed report from the FAA investigator, Allegiant and its maintenance contractor, AAR, failed to perform procedures that would have caught the error no less than five times. The report called it "a deliberate and systemic act of non-compliance" that had endangered thousands of passengers on more than 200 subsequent Allegiant flights.

The inspector recommended strong enforcement action and maximum fines. But superiors, citing the new compliance philosophy, ignored the recommendations and closed the case.

Loretta Alkalay: Actually, the file called for a much larger investigation, but the FAA just said-- basically letter of correction, which means nothing.

Steve Kroft: So, it wasn't even a slap on the wrist.

Loretta Alkalay: Right.

Steve Kroft: Or barely a slap on the wrist.

Loretta Alkalay: Well letters of correction are nothing, they are not enforcement action. There's no record. I mean, the FAA knows that there's a letter of correction, but you can't use it to augment a sanction. They're just-- they're basically nothing.

Steve Kroft: Are you aware that your own investigator recommended a heavy fine?

John Duncan: Yes.

Steve Kroft: And yet, to our knowledge, there was no enforcement action. No fine.

John Duncan: So as we--

Steve Kroft: Nothing other than what's been described to us as not even a slap on the wrist. That was a major screw up, wasn't it?

John Duncan: This was certainly a major event. And-- and so our charge in these kinds of events is to assure that they don't happen again.

Steve Kroft: This was three years ago. I mean this was 2015. And we've had all these other incidents, these 100 incidents, that have occurred since then. It seems like they're on top of it?

John Duncan: All those incidents have been addressed as I've described multiple times.

Back in 2015, the only people that seemed to be paying attention to Allegiant were reporters at the Tampa Bay Times. The airline has a major hub in St. Petersburg and the paper began keeping track of Allegiant's missteps, digging into records and documenting the potential dangers. The coverage prodded the FAA to undertake a thorough review of Allegiant's operations in April of 2016. Three months later, the FAA concluded there were no serious deficiencies, recertified the airline for five more years and says it has been monitoring ever since without noticing any systemic problems.

But last July, while Allegiant was having a very bad month, we decided to follow up where the Tampa Bay Times had left off to see if the airline had improved its operations. We filed a Freedom of Information request with the FAA asking for more than a year's worth of mechanical interruption summary reports from Allegiant and seven other airlines so we could make a comparison. We received the documents for every airline except Allegiant, which objected to their release.

Steve Kroft: What do you make of that?

John Goglia: Well, obviously, they have something to hide. And you have-- a number of 'em from other airlines, a whole stack from United Airlines. Calls into question why Allegiant is stopping it. You know, is there something there that they don't want to see? Is there something there that the FAA doesn't want you to see, either? So that actually points at both of 'em.

Steve Kroft: Seven other airlines had no problem with us looking at their records.

John Duncan: I appreciate that.

Steve Kroft: Only Allegiant.

John Duncan: I have no idea what their rationale is in that regard.

Six days after this interview, the FAA overruled Allegiant's objections and produced the documents. They showed, on average, the airline was nearly three and a half times more likely to have mid-air breakdowns than American, United, Delta, JetBlue and Spirit.

"If, God forbid, there is an accident, I think there will be a lot of people saying, 'Well, we knew. We knew and we did nothing.'"

But even more disturbing are new allegations from the ranks of Allegiant's own pilots. Their union president, Captain Daniel Wells, says he's concerned that Allegiant is trying to gain a competitive cost advantage by softening safety standards adhered to by the major airlines. That pilots are being told to think twice before declaring costly emergencies. And that Allegiant's maintenance department tries to talk pilots out of reporting problems with their aircraft to avoid delays and keep the planes moving.

Daniel Wells: What I hear from the Allegiant pilots are-- they get-- a call from maintenance control, from-- who is-- an agent of the company and says, "Y-- you didn't write anything up, did you?" Meaning you didn't notice any maintenance problems on the airplane. And that's a very clear-- message to send to pilots that the company is discouraging you from-- recording maintenance deficiencies.

Steve Kroft: Is that legal?

Daniel Wells: No, because our captains are required to report any mechanical deficiencies of an aircraft.

In response, Allegiant's statement to us says, in part:

"Any employee who fails to report safety-related concerns through available channels is in violation of company policies, and may also be in violation of federal regulations."

Steve Kroft: The head of the pilots' union told us that Allegiant's maintenance operation is discouraging pilots from reporting mechanical difficulties on the flights. Would that alarm you?

John Duncan: Certainly discouraging pilots from reporting-- legitimate-- maintenance problems-- would concern me a great deal.

Steve Kroft: That's against the law, isn't it? I mean--

John Duncan: It's certainly against--

Steve Kroft: --aren't pilots required to report this?

John Duncan: They are. It certainly doesn't meet the safety standards that we would anticipate.

Steve Kroft: Have you ever heard this before?

John Duncan: I have not.

Steve Kroft: Is that something you might look into?

John Duncan: It's something that we do look into on a routine basis. Yes.

Steve Kroft: Is it something you will look into?

John Duncan: It's something we will continue to look into.

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It must be noted that there has been a sharp drop-off in the number of Allegiant's service difficulty reports since we notified the airline and the FAA that we were working on this story and began requesting information. It may have to do with the fact that Allegiant replaced ten of its old MD-80s with new aircraft from Airbus. But we do know that this serious incident that occurred in September is not reported in the public record.

Allegiant Flight 514 had just landed in Fresno, California and was rolling to the gate, when it suddenly stopped short as the cabin began to fill up with fumes and smoke. Scott Shuemake, his brother, Chris, and sister-in-law, Shanyl, were on the plane.

Scott: I'll never forget this the most absurd thing I've ever heard in my life. Captain comes on and says, "Ladies and gentlemen, we've been informed there's-- there's smoke in the cabin. Please start breathing through--"

Chris: "Your shirts."

Steve Kroft: Through your shirt?

VARIOUS: Yeah.

Steve Kroft: Was it hard to breathe?

VARIOUS: Yeah. Oh yeah.

Shanyl: You couldn't.

Chris: The crying flight attendants walking around kind of blindly--

Scott: Not making eye contact.

Chris: Yeah, not making eye contact, blindly handing out wet cocktail napkins.

Shanyl: She's just crying.

Chris: And saying, "Breathe through this."

The oxygen masks in the cabin didn't deploy and the passengers say minutes went by before the crew moved to get them off the plane.

Scott: We're like, "Open the damn door. We just need some fresh air. Like, it's okay that, you know, there's a problem. You guys can take your time to figure out what the problem is, but open the door."

Steve Kroft: So, how long were you on the plane?

Chris: 12-15 minutes.

Scott: Yeah. Literally just-- breathing this-- this white substance.

The captain eventually dropped the stairs at the back of the plane and passengers evacuated one by one and walked across the taxiway to the terminal. But not before receiving one last instruction from the cabin crew.

Shanyl: They said, "Make sure you take--"

Chris: --so then they-- yeah, they told--

Shanyl: "--your carry-ons."

Chris: --come on, say, "Take your carry-ons and exit the plane out the rear."

Allegiant has yet to tell passengers what they were breathing, but the airline confirmed to us that the fumes were from Skydrol 4, a hazardous hydraulic fluid. In a brief statement the day of the incident, Allegiant said only that "a mechanical issue arose that caused a visible haze to appear". Quote, "passengers deplaned with any carry-on items" and "proceeded to the terminal"…"per normal procedures."

Scott: They can't have the image on the 6:00 news of a bunch of passengers jumping with their-- with their arms crossed on a yellow-- on a yellow slide. The fact that they made such a big deal about reminding us to take our carry-on bags as we're getting off the plane. In--

Shanyl: So it'd look normal.

Scott: Yeah. There was nothing normal about the way we (LAUGH) deplaned that aircraft.

Steve Kroft: Do you think the general public is--

Loretta Alkalay: No.

Steve Kroft: --aware with the issues involving Allegiant?

Loretta Alkalay: No. People believe that if they hold a certificate and they're flying, they must be safe. The FAA's on it.

Steve Kroft: But the FAA is not on it, you say.

Loretta Alkalay: It does not appear that they are on it when it comes to Allegiant, no.

Most of the public is also unaware that Allegiant's CEO Maurice Gallagher was one of the founders of Valujet, another low-cost carrier with the same business model as Allegiant. Valujet never recovered from the crash of one of its planes in the Everglades in may of 1996. Loretta Alkalay, like the former NTSB member, John Goglia, says she would never fly Allegiant.

Steve Kroft: Do you know anybody in the industry that-- that flies Allegiant?

Loretta Alkalay: No. (LONG PAUSE) No. And I know that a lot of people talk about how they don't fly Allegiant, so it's very concerning. I know people that worked at the FAA who say they would never fly Allegiant.

Steve Kroft: I mean, that's quite an admission. I mean, this just seems like one of those secrets that everybody knows, and then if you have a plane go down, it'll all come out.

Loretta Alkalay: You know, if, God forbid, there is an accident, I think there will be a lot of people saying, "Well, we knew. We knew and we did nothing."

Produced by Michael Karzis and Vanessa Fica.