Why Airliners Make Strange Noises (And the One Noise That Really Scared Us)!

Added: 2026-05-21

[00:00:00]I'm going to tell you about noises that are normal but concern pastors. And a very loud noise I once heard that concerned us very much, but apparently only the pastures sitting directly over the event in the wingroot area. Only they got concerned. Everybody else should have been though. It was an interesting event. First of all, you're sitting on an airliner at night. I'm going to give you an example about one of these noises. Cabin lights dim.

[00:00:24]Suddenly, you hear barking noises underneath the airplane. A loud hydraulic wine starts coming from the wing. The flaps look like they're practically disassembling themselves.

[00:00:34]And right after takeoff, the engines suddenly sound quieter instead of louder. Most passenger think that can't possibly be normal. But after 55 years of flying, I can tell you something surprising. Some of the strangest noises on an airliner are actually signs that everything is working perfectly. And tonight, we're going inside the cockpit to explain what those sounds really are.

[00:01:00]One thing I've learned over decades of airline flying is this. Passengers judge airplanes mostly by sound. Pilots judge them by systems. And modern airliners are constantly talking to you through noises. Hydraulics, pumps, electric motors, fans, actuators, gear doors, pressurization systems. The funny part is the sounds pastures worry about most are usually completely normal.

[00:01:25]Meanwhile, the truly dangerous situations are often eerily quiet. So, today I want to talk uh I want to walk you through some of the strange noises people hear on airliners, what causes them, and what really happens behind the scenes. Now, the barking dog sound.

[00:01:44]Let's talk about one of the most famous airline noises of all. You're sitting on an Airbus at the gate. You've just pulled it in and park. The captain is quiet. Then suddenly, bark, bark, bark.

[00:01:54]It honestly sounds like there's a mechanical dog trapped underneath the airplane. And if you ever And if you've never heard it before, it can get your attention in a hurry. Pastors stop talking. People start looking around nervously. And nervous flyers immediately think that can't possibly be normal. Now, after 55 years of flying, I've learned something interesting. The noises passengers worry about the most are usually signs that the airplane is functioning properly and I had a funny experience with that uh one time sitting in the back of the Airbus. I was fully qualified on the airplane so I knew what was going on. And I was commuting in civilian clothes, just another passenger sitting in the cabin like anybody else. And this lady next to me suddenly hears bark bark bark. It actually sounds more like But I digress. She looks over at me with his worried expression and says, "What is that noise?" Now, fortunately, I knew exactly what it was. So, I explained to her that on Airbus airplanes, there's a system called the power transfer unit or PTU. Pilots jokingly call it the barking dog. Okay. What's happening is this.

[00:03:01]Modern Airbus aircraft have multiple hydraulic systems and the PTU helps transfer hydraulic pressure between systems uh whenever one side needs pressure assistance. What creates the barking sound is the PTU, the power transfer unit cycling rapidly on and off as the hydraulic pressure changes. So instead of one smooth electric motor sound, you get this rhythmic bark bark bark. Perfectly normal. In fact, it's actually a sign that the airplane's backup systems are functioning exactly as designed. Well, after I explained this all to her, she looked at me kind of suspiciously and asked, "Are you a pilot?" Now, at the time, there was this old Holiday in Express commercial running on television, the one where people stayed at Holiday in Express and suddenly became experts in everything, doctors, surgeons, engineer. So, I looked at her and said, "No, but I stayed at a Holiday and Express last night." And fortunately, she laughed.

[00:03:56]But honestly, moments like that have taught me something important.

[00:04:00]Passengers hear all kinds of strange noises on airliners. But most of the time, those noises are simply the sound of incredibly sophisticated systems doing exactly what they were designed to do. Now, here's a sound that uh gets the passengers attention. You're pushing back from the gate. Then suddenly, clunk clunk. People look around nervously.

[00:04:22]What was that? Usually, it's just the toeb bar flexing as the tug uh changes direction or takes up the load. Now, we're talking about anything from a couple hundred,000 lb aircraft to uh my trip 7, which was would be close to 745,000 lbs are pushing back. So, that that's a lot of stress on that that toe bar. And there's a tremendous amount of force involved in moving an airliner around the lamp. the ramp, the toeb bar flexes slightly that releases tension in a loud mechanical clunk. Totally normal.

[00:04:54]But sitting in the back as an offduty pilot, I used to watch passengers faces every time that happened. You could almost hear them thinking, "Well, that sound didn't sound good." Meanwhile, the pilots and mechanics up front barely even notice it. To them, that's just a normal push.

[00:05:12]Okay. Now, every airplane has its own personality when it comes to sounds. All the Boeing 727s had some unforgettable ones, especially the flax flap system.

[00:05:24]Because honestly, when those flaps start moving, it sounds like the airplane was taking itself apart. Now, in the 727, the flaps were driven through a screw jack that's in the little box up there.

[00:05:36]And the screwjack mechanism is powered by a hydraulic motor. And when those flaps start extending, you know it, that little motor goes like that. Okay? And it goes on for quite a few seconds uh running the flaps back. So you hear this mechanical wine and the flaps uh are moving slowly and the noise would suddenly stop for a moment at like 2° of flaps, 5 degrees of flaps, 15. Now it's quiet. Then a few seconds later, we're again. Okay, you're lowering to the next flap section. And if you're sitting near the wing, it honestly looked like the airplane was disassembling itself. Those fowler flaps, flap tracks moving, panel shifting, mechanisms extending, huge sections of wing opening up. Passengers used to get very concerned watching all that happen, especially at night. I can remember sitting in the back commuting or deadheading and watching people stare out the window with this worried expression like, "Should all those parts really be moving around like that?" But the reality is those flap systems are engineering masterpieces. The reason airliners can take off slowly and land safely at heavy weights is because those flap systems and they dramatically change the shape of the wing. And yes, they make a lot of noise doing it.

[00:06:54]Another sound passengers notice all the time is the famous whining noise after engine start. You hear this wee. That's often hydraulic pumps, electrical systems, or the air conditioning packs.

[00:07:05]Modern airliners use bleed air from the engines. Hot compressed air powers all kinds of systems throughout the airplane. And honestly, airliners are close to flying are closer to flying power plants than most people realize.

[00:07:19]The airplane is constantly balancing air flow, pressure, temperature, electrical loads, and hydraulic systems. And every one of those systems has a specific sound attached to it.

[00:07:30]Now, this one scares people constantly.

[00:07:32]You look out the the window during turbulence and the wing is bouncing around dramatically. Passengers think that wing is going to break off. Nope.

[00:07:41]Actually, if the wing wasn't flexing, I'd be worried. Modern airline wings are designed to bend. That flexing absorbs energy. The rigid wing would crack if it didn't flex. But a flexible wing survives. And during certification testing, engineers literally bend wings upward to incredible angles before failure, far beyond anything you'd ever see in a normal airline operation. So when you see those wings flexing around turbulence, that's not weakness, that's strength. I remember the first time I'm chasing uh the NASA B-52. I'm out on the wing. I'm in a T38 and I'm trying to maintain a formation position. That wing is flexing up and down. It flexes actually 17 feet. So, I'm off the side and I'm trying to stay in position. I got this airplane flapping wing out there. That was one of my first uh initiations into how much these wings could flap. Now, let's talk another one that uh confuses almost everyone. The airplane starts the takeoff roll. The engines roar. Then, after liftoff, the sound decreases slightly. Passengers think, "Did we lose an engine?" Usually, no. That simply is thrust reduction.

[00:08:52]During takeoff, pilots use very high power settings, but once airborne, the airplane no longer needs maximum thrust or near maximum thrust if you're doing a reduced thrust takeoff. So, the automation reduces power to climb thrust. The engines get quieter.

[00:09:07]Perfectly normal. But if you don't know that, it can definitely get your attention. Now, having said that, every once in a while, you hear a noise that gets the crews attention immediately.

[00:09:18]And I remember one on the Boeing 727 that I'll never forget. We were flying from Chicago down to Indianapolis. Just a quick turn out and back. Didn't even bring a suitcase. Big mistake. As we were climbing out, suddenly, kaboom, womph. A loud bang from underneath the airplane. Now, this is different. It wasn't a normal hydraulic sound. It wasn't flat movement. It wasn't gear doors. And it immediately got our attention. The captain and I looked at each other instantly, and we started checking everything. hydraulics, electrics, pressurization, engine instruments. We're looking around going, "What was that? Everything looks normal." Got our attention, but we both knew something wasn't right. Then the flight attendant called up. She happened to be standing near the wing area. Uh, and she said, "What was that noise?"

[00:10:06]Really got the attention of a couple passengers and her that were right in that area. She said it scared the heck out of them back there. Now, here's the interesting part. Most of the other passengers apparently never even noticed it. Passengers will often react to perfectly normal noises, but this one actually was not a good noise and many of them missed it completely. But the flight attendants noticed, and we sure noticed. So, we continued into Indianapolis after we parked. The captain sent our second officer outside [snorts] to take a look around. Now, I was a co-pilot back then, and a few minutes later, she comes back in and says, "Everything looks fine." Well, the captain and I looked at each other and we both thought the same thing. No, something's wrong. So, we both went outside and looked for ourselves. And underneath the airplane, near the air conditioning packs, um, right in front of where the gear doors are in the image there, um, we discovered two large panels missing from the bottom of the aircraft, about 2 feet by 2 feet each, gone, completely torn off. There was still a little bit of hinge material kind of hanging there, looking sharp. uh where they had ripped away and what we suspected happened was that maintenance probably hadn't secured the panels properly and somewhere during a climbout they departed the airplane. Now fortunately it didn't damage anything important that we could tell but suddenly we had two large holes in the bottom of the airplane and the airplane wasn't going anywhere. So there we were stuck in Indianapolis. No suitcase, no overnight gear, nothing, not even a toothbrush. By the time maintenance got coordinated, they realized this is not going to be a quick repair. Well, we kind of realized that right away. Wasn't a surprise to us. So, we ended up taking our epilelettes off our uniform shirts and we went out to dinner at a TGIF Fridays and three of us sitting there in our blue pants and our white shirts eating dinner. Okay. Yeah, I remember that. And uh we were spending the night there un uh unexpectedly without even a change of underwear. That was the last time I ever went on a trip without bringing at least a small overnight bag.

[00:12:11]I learned my lesson that day. The next day, we finally they finally did it ahead of us home because the airplane wasn't going to get fixed anytime soon.

[00:12:20]But that experience taught me something important. Pilots learned to recognize the difference between interesting noises and that definitely should have h should not have happened. Most sounds on airliners are completely normal, but occasionally the airplane is really trying to tell you something.

[00:12:40]And there are all these random clicks, hums, mechanical noises during cruise.

[00:12:44]Trim motors, valve actuators, cargo cooling fans, hydraulic pumps, cycling, recirculation fans. Airliners are filled with electric motors and moving systems.

[00:12:56]And many of them are physically very close to the passenger cabin. 35,000 ft.

[00:13:01]When outside temperature is around -60 degrees Fahrenheit, hundreds of systems are quietly working together to keep everyone alive and comfortable. You're hear you're hearing the airplane operate in real time.

[00:13:17]One thing over 55 years of flying has taught me is this. Airliners are noisy because they're busy. Thousands of components are operating simultaneously.

[00:13:26]hydraulics, electrics, pneumatics, fuel system, flight controls, pressurization systems, environmental systems, all working together continuously. The airplane is constantly communicating with you through sounds. The irony is passengers often get nervous during completely normal operations when the pilots are sitting up front thinking, "This airplane is performing exactly as designed. We're having our nice little cup of coffee and just enjoying the flight." Now, obviously, pilots pay attention to unusual sounds. But occasionally, noises do matter, but most of the strange sounds people hear every day, they're simply the sounds of highly sophisticated machine doing exactly what it was designed to do. So, the next time you're sitting in an airliner and you hear barking, whining, chunking, hydraulic noises or the wing flexing around in turbulence, you might remember this video because most of the time those strange noises are exactly the signs that the airplane is healthy. And after 55 years of flying, I can tell you something reassuring.

[00:14:27]The airplanes that the worried me the most were usually the quiet ones. I'm Captain Rod and thanks for flying along.

[00:14:35]I hope you enjoyed it.