# What's that "roar" when a commercial airliner lands? **URL:** \ **Category:** Factual Questions\ **Created:** [October 28, 2004, 5:14pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691 "2004-10-28T17:14:05Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![The\_Chao\_Goes\_Mu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_chao_goes_mu/32/3191_2.png) [@The\_Chao\_Goes\_Mu](https://boards.straightdope.com/u/The_Chao_Goes_Mu)\ **Post date:** [October 28, 2004, 5:14pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/1 "2004-10-28T17:14:05Z")
While waiting for my GF at Detroit Metro Airport last week, I was sitting out on the top deck of the parking garage watching take-offs and landings. (I could do that for hours) and one thing I’ve noticed as I have in the past, even while as a passenger, is that during the landing, after the plane touches down there is an incredible roar just as the plane is rapidly slowed on the runway. It’s almost deafening if you’re close enough to the runway (as I was.) Are these reverse thrusters? The larger the aircraft, the louder the roar and they all hit the strip at the same spot and began their “braking” right in front of me. I’ve already checked this thread out and although informative, it didn’t answer my specific question. http://boards.straightdope.com/sdmb/showthread.php?t=172817&highlight=airplane+brakes ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [October 28, 2004, 5:20pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/2 "2004-10-28T17:20:48Z")
Yes it is reverse thrust, because on selection of reverse thrust, the actuation system folds the blosker doors to blank off the cold stream final nozzle, thus diverting the airflow through the cascade vanes. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 5:22pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/3 "2004-10-28T17:22:52Z")
> [@The Chao Goes Mu](#): > > Are these reverse thrusters? Yep. Also, I believe (and could be mistaken) that the pilot will throttle up slightly after deploying the reversers to amximize the effect. They aren’t “reverse thrusters”, _per se_, but are baffles that are moved hydraulically to cover the exhaust of the engines and direct the thrust with a forward vector. ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [October 28, 2004, 5:25pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/4 "2004-10-28T17:25:00Z")
Jet engines has a thrust reversal mechanism that works a lot like an umbrella. The pilot throws a switch and clamshell-shaped panels unfold to catch the exhaust of the engine, deflecting it forward and slowing down the plane. The roar you hear is the pilot opening up the engines to full-throttle, the better to produce more thrust, thus slowing the plane down faster. The added benefit is that if something goes wrong with the landing, the plane has enough thrust to ascend again. ---
**Author:** ![The\_Chao\_Goes\_Mu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_chao_goes_mu/32/3191_2.png) [@The\_Chao\_Goes\_Mu](https://boards.straightdope.com/u/The_Chao_Goes_Mu)\ **Post date:** [October 28, 2004, 5:27pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/5 "2004-10-28T17:27:36Z")
🆒 Thanks! ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [October 28, 2004, 5:32pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/6 "2004-10-28T17:32:22Z")
The mechanism is called a “thrust reverser” not a reverse thruster. Here’s a decent pic of one: [http://sitemaker.umich.edu/thrust.reverser/see\_thrust\_reverser\_deployed\_part\_b](http://sitemaker.umich.edu/thrust.reverser/see_thrust_reverser_deployed_part_b) ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [October 28, 2004, 5:56pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/7 "2004-10-28T17:56:43Z")
- - - The concept of engine thrust reversers is something I have never quite understood, in that atmospheric engines produce thrust not so much by “pushing air out their rear ends” but by “vacuuming air from their front ends”. To produce thrust by rearward expulsion, they would need convergent+divergent nozzles, which they do not have; subsonic jet engines always have plain convergent nozzles. Which means that… it doesn’t matter _which direction_ the rear end nozzles are pointed, as long as the engine is still spinning it will be drawing air in the front, producing forward thrust… ? Seems to me that if you wanted to slow down, it would be better to just stop the engine ASAP. ~ ---
**Author:** ![The\_Chao\_Goes\_Mu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_chao_goes_mu/32/3191_2.png) [@The\_Chao\_Goes\_Mu](https://boards.straightdope.com/u/The_Chao_Goes_Mu)\ **Post date:** [October 28, 2004, 5:57pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/8 "2004-10-28T17:57:48Z")
Are they all pretty much the same? I didn’t even notice it on the planes when they were landing. Although, that may be due to my not actually looking for anything like that. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 6:01pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/9 "2004-10-28T18:01:15Z")
> [@DougC](#): > > The concept of engine thrust reversers is something I have never quite understood, in that atmospheric engines produce thrust not so much by “pushing air out their rear ends” but by “vacuuming air from their front ends”. No, this is incorrect. The engines do _not_ “pull” the airplane through the air, they _push_ it. The thrust comes from the burning of fuel, which produces a large volume of hot gas. The net result is that there is a much greater volume of gas being expelled from the rear of the engine than is being drawn in from the front. This is what produces the thrust. ---
**Author:** ![av8rmike](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@av8rmike](https://boards.straightdope.com/u/av8rmike)\ **Post date:** [October 28, 2004, 6:23pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/10 "2004-10-28T18:23:12Z")
> [@Q.E.D.](#): > > No, this is incorrect. The engines do _not_ “pull” the airplane through the air, they _push_ it. The thrust comes from the burning of fuel, which produces a large volume of hot gas. The net result is that there is a much greater volume of gas being expelled from the rear of the engine than is being drawn in from the front. This is what produces the thrust. Sorry bud, but this is incorrect as well. The volume of air flowing through the engine (bypass air and combustion air) has to be equal on the inlet and outlet sides, otherwise the engine would somehow be _adding_ air to the system. What increases from inlet to outlet is the air’s velocity and temperature, but the overall volumetric flow is the same. ---
**Author:** ![fortytwo](https://avatars.discourse-cdn.com/v4/letter/f/f9ae1b/32.png) [@fortytwo](https://boards.straightdope.com/u/fortytwo)\ **Post date:** [October 28, 2004, 6:34pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/11 "2004-10-28T18:34:55Z")
> [@The Chao Goes Mu](#): > > Are they all pretty much the same? I didn’t even notice it on the planes when they were landing. Although, that may be due to my not actually looking for anything like that. Thrust Reversers vary from aircraft to aircraft. A/c with high by-pass engines generally block and divert cold stream air only. Older types with low by-pass engines often block and divert hot exhaust air. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 6:38pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/12 "2004-10-28T18:38:52Z")
> [@av8rmike](#): > > Sorry bud, but this is incorrect as well. The volume of air flowing through the engine (bypass air and combustion air) has to be equal on the inlet and outlet sides, otherwise the engine would somehow be _adding_ air to the system. What increases from inlet to outlet is the air’s velocity and temperature, but the overall volumetric flow is the same. The engine _is_ adding gas (note that I said “gas” not “air”) to the system. A small volume of burned liquid fuel turns into a large volume of hot exhaust gas. As you say, the velocity of this exaust is much higher than the intake which has the result of lowering the pressure somewhat (otherwise the flow of gas would not be directional). The temperature, also as you say, is increased. If you remember your gas laws from high-school chemistry class, you’ll know that if the temperature increases and the pressure decreases, then the volume _must_ increase. It cannot be otherwise. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [October 28, 2004, 6:42pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/13 "2004-10-28T18:42:07Z")
> [@av8rmike](#): > > Sorry bud, but this is incorrect as well. The volume of air flowing through the engine (bypass air and combustion air) has to be equal on the inlet and outlet sides, otherwise the engine would somehow be _adding_ air to the system. What increases from inlet to outlet is the air’s velocity and temperature, but the overall volumetric flow is the same. Sorry bud but you are ignoring the effects of compression and expansion. Air enters the engine. The air is compressed. fuel is added. Fuel is lit. BOOM! Gasses expand, and exhaust pushes out the back of the engine. Plane goes forward. Of course this ignores the turbines and how the air is compressed etc, but it is close enough. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 6:47pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/14 "2004-10-28T18:47:07Z")
> [@Rick](#): > > Fuel is lit. BOOM! I’m glad you didn’t design the engines on the planes I’ve flown on. 😉 ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [October 28, 2004, 6:57pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/15 "2004-10-28T18:57:41Z")
> [@DougC](#): > > atmospheric engines produce thrust not so much by “pushing air out their rear ends” but by “vacuuming air from their front ends”. Not true. They produce thrust by expelling gas with a much higher energy content than the air they take in. F=ma and all that. > [@](#): > > To produce thrust by rearward expulsion, they would need convergent+divergent nozzles, which they do not have; subsonic jet engines always have plain convergent nozzles. Not true and not true; simply expelling higher-energy exhaust does the trick. The divergent section of a nozzle is not required, it simply adds efficiency. > [@](#): > > Which means that… it doesn’t matter _which direction_ the rear end nozzles are pointed, as long as the engine is still spinning it will be drawing air in the front, producing forward thrust… ? Reversers work simply by redirecting the exhaust in a partly-forward direction (and partly outboard; it’s much simpler and lighter to do that). Most modern high-bypass turbofans produce most of their thrust from the cooler bypass air moved only by the fan, and their reversers work only on the fan. The forward thrust from the core remains, but isn’t worth going after (the C-17 is an exception because it has to taxi backwards up a grade with a full load). One way has the outer duct remaining fixed with an array of blocker doors folding into the fan discharge, diverting the flow into a cascade resembling a Venetian blind, angling the air forward. With the engines back up to full power with the reversers deployed (they’re at idle or close to it during descent and touchdown), the reverse thrust can be 40% or so of the max forward thrust. Many newer high-bypass turbofans dispense with the blocker doors and have the aft portion of the cowl translate aft, with the flowpath geometry shaped so that the duct is closed off. This also exposes a vane cascade as in the above. Older low-bypass or straight-pipe engines usually have a folding clamshell arrangement that blocks and redirects the entire exhaust. Reverser deployment in-flight is seriously bad and has caused a few crashes. That’s usually prevented by the “squat switch”, which senses weight-on-wheels and keeps electrical things from happening at the wrong times, but they’re never 100% reliable. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [October 28, 2004, 6:58pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/16 "2004-10-28T18:58:46Z")
> [@Q.E.D.](#): > > I’m glad you didn’t design the engines on the planes I’ve flown on. 😉 But I probably have. Be afraid. Be very afraid. ---
**Author:** ![av8rmike](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@av8rmike](https://boards.straightdope.com/u/av8rmike)\ **Post date:** [October 28, 2004, 7:00pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/17 "2004-10-28T19:00:13Z")
> [@Q.E.D.](#): > > if the temperature increases and the pressure decreases, then the volume _must_ increase. It cannot be otherwise. OK, but the _mass_ flow remains constant, since the greatly increased volume and reduced density balance out. To **DougC** : Thrust reversers work the way they do because of conservation of momentum. By re-directing the thrust outwards instead of rearwards, the direction of the force is changed without having to actually reverse the spin of the engine or “close off the end” or anything. If re-directing the nozzle didn’t change the direction of the force vector, VTOL’s like the Harrier wouldn’t be possible! ---
**Author:** ![Jurph](https://avatars.discourse-cdn.com/v4/letter/j/50afbb/32.png) [@Jurph](https://boards.straightdope.com/u/Jurph)\ **Post date:** [October 28, 2004, 7:08pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/18 "2004-10-28T19:08:45Z")
> [@The Chao Goes Mu](#): > > Are they all pretty much the same? I didn’t even notice it on the planes when they were landing. Although, that may be due to my not actually looking for anything like that. You won’t usually be able to see them. On a high-bypass turbofan mounted under a wing, the thrust reverser is usually hidden inside its own [cowling](http://www.sandford.org/gandercrash/investigations/majority_report/graphics/figure_1-7.jpg) (see link) that is only visible from a few seats on the plane. When it opens, hot gas is diverted up over the leading edge of the wing, and to the sides, and possibly downward as well. [Here](http://www.diff.net/media/2003_03_22_traveling/img_5598.htm) is a view of the slightly forward-angled grating from directly above the wing, looking down. The plane is facing the right of the photo, and you’re looking down onto the plane’s left wing. [Here’s a shot](http://www.diff.net/media/2003_03_22_traveling/img_5600.htm) of the same thrust reverser in the normal (stowed) position. And here’s yet another view of a [thrust reverser](http://home.swipnet.se/~w-26408/211rev1.jpg) – this is the #3 engine of an L-1011, viewed from below. The exhaust gases come out of the slotted vent that’s about midway along the length of that engine. Most modern airliners use a design similar to that one. By the way, if you ever want a lesson in high-reliability parts, ask an engineer about the design of a thrust reverser. The two things a thrust reverser must never do are accidentally deploy in flight (mid-air cartwheel) and fail to deploy when commanded (low-altitude runway-kisser cartwheel). ---
**Author:** ![Jurph](https://avatars.discourse-cdn.com/v4/letter/j/50afbb/32.png) [@Jurph](https://boards.straightdope.com/u/Jurph)\ **Post date:** [October 28, 2004, 7:11pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/19 "2004-10-28T19:11:24Z")
> [@av8rmike](#): > > OK, but the _mass_ flow remains constant, since the greatly increased volume and reduced density balance out. Remember that the engine is not a closed system – there is fuel being pumped into the airflow. The magic of thermodynamics means that a very very small increase in exit _mass_ can still create a very large difference in exit _energy_. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 7:11pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/20 "2004-10-28T19:11:54Z")
> [@ElvisL1ves](#): > > But I probably have. Be afraid. Be very afraid. Well, I built some of the electrical system components. So _you_ be afraid. 😛 [Next page](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691.md?page=2)