# 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:** 2
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [October 28, 2004, 7:17pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/21 "2004-10-28T19:17:22Z")
> [@Jurph](#): > > You won’t usually be able to see them. Would I be right in thinking that at least some designs are highly visible - I think it’s a 737-200 that I’ve been sat in a rear window seat and seen large semi-cylindrical section swing back and into an angle behind the engine. ---
**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, 7:46pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/22 "2004-10-28T19:46:10Z")
One particular design used on older low-bypass turbofans, such as those on MD-80s and other aft-mounted engines had a particularly visible design. The entire aft cowling would split in two and swing backwards until it formed a sort of “umbrella” shape. You had to be watching from outside the aircraft, as there aren’t any windows or seats that far aft. ---
**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, 7:54pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/23 "2004-10-28T19:54:46Z")
> [@GorillaMan](#): > > Would I be right in thinking that at least some designs are highly visible - I think it’s a 737-200 that I’ve been sat in a rear window seat and seen large semi-cylindrical section swing back and into an angle behind the engine. Highly possible. They are similiar to but not quite the same as those in the link in post #6 ---
**Author:** ![Acsenray](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/acsenray/32/4519_2.png) [@Acsenray](https://boards.straightdope.com/u/Acsenray)\ **Post date:** [October 28, 2004, 8:01pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/24 "2004-10-28T20:01:38Z")
> [@don't ask](#): > > 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. Maybe it’s exactly true, I don’t know, but this almost sounds like a joke. ---
**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, 8:18pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/25 "2004-10-28T20:18:47Z")
Actually it’s all true (apart from the **blosker** doors) . Should be **blocker** 🙂 ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [October 28, 2004, 8:33pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/26 "2004-10-28T20:33:30Z")
> [@Vetch](#): > > Highly possible. They are similiar to but not quite the same as those in the link in post #6 That’s the one - somehow I missed that link :smack: ---
**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, 8:39pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/27 "2004-10-28T20:39:02Z")
> [@Q.E.D.](#): > > > [@Rick](#): > > > > Fuel is lit. BOOM! > > I’m glad you didn’t design the engines on the planes I’ve flown on. 😉 ::: Sigh::: I’m at work, short on time so I cut to the chase. Should have know this bunch wouldn’t let me slide. :smack: Anyway I don’t design engines, I teach guys how to work on them. The four cycles in an Otto cycle engine are: Suck Squeeze Bang! Whoosh So you guys be very afraid, the guy working on your car was taught by me! ---
**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, 9:41pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/28 "2004-10-28T21:41:39Z")
> [@](#): > > 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. - QED - - - Consider this: a jet engine is at least three sections: a compressor, a combustor and an exhaust turbine stage. If the pressure/temperature from the exhaust-section of the engine was ever greater than the pressure from the intake section, then air would start flowing _backwards_ through the engine. > [@](#): > > If re-directing the nozzle didn’t change the direction of the force vector, VTOL’s like the Harrier wouldn’t be possible! - Yes it is true that you can generate thrust just by expelling gas–but it is enormously inefficient, and as somebody pointed out, large-jet turbofans usually only divert their fan rear flows with reversers. They don’t even bother with the turbine exhaust, as it is so small it is seen as insignificant. My point is that since the forward thrust is really occurring in front of the fans, if you really wanted to stop the fans from generating forward thrust you would be better off stopping air from getting into the fans from the _front_ than you would by doing anything with the air getting blown out the _rear_. ~ ---
**Author:** ![ScoobyTX](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@ScoobyTX](https://boards.straightdope.com/u/ScoobyTX)\ **Post date:** [October 28, 2004, 10:45pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/29 "2004-10-28T22:45:33Z")
> [@Rick](#): > > The four cycles in an Otto cycle engine are: > Suck > Squeeze > Bang! > Whoosh Gas turbine engines don’t use the Otto cycle. They use the [Brayton cycle](http://www.lerc.nasa.gov/WWW/K-12/airplane/brayton.html). ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [October 28, 2004, 11:05pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/30 "2004-10-28T23:05:44Z")
But… You don’t want to stop generating forward thrust just yet, in case you need to TOGA (take off and go around) or otherwise abort the landing, which is why aircraft generally touch down with the engines at or close to full throttle. I’m not a pilot, but the landing sequence is generally along the lines of: Approach runway at low speed with full flaps (to lower the stall speed) Flare Push throtles up \* Deploy reversers \* Touch down Commit to landing Cut engine power Apply wheel brakes - Something that hasn’t really been touched yet is that in addition to the increased noise simply from running up the thrttle, the reversers add their own noise as they disrupt the exhaust flow from the engines. ---
**Author:** ![JRDelirious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jrdelirious/32/9531_2.png) [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)\ **Post date:** [October 28, 2004, 11:41pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/31 "2004-10-28T23:41:09Z")
> [@gotpasswords](#): > > But… You don’t want to stop generating forward thrust just yet, in case you need to TOGA (take off and go around) or otherwise abort the landing, which is why aircraft generally touch down with the engines at or close to full throttle. > > I’m not a pilot, but the landing sequence is generally along the lines of: > > Approach runway at low speed with full flaps (to lower the stall speed) > Flare > Push throtles up \* > Deploy reversers \* > Touch down > Commit to landing > Cut engine power > Apply wheel brakes > > - Something that hasn’t really been touched yet is that in addition to the increased noise simply from running up the thrttle, the reversers add their own noise as they disrupt the exhaust flow from the engines. Whoa, there… in every commercial landing I’ve been in, I have most definitely felt the mains rolling on the surface (sometimes after one or two bounces) _before_ feeling by either noise, vibration or deceleration that reverse thrust was applied. Is that an illusion? ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [October 28, 2004, 11:44pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/32 "2004-10-28T23:44:01Z")
I suppose I should add to this thread that I have once flown on an airline where they gave an explanation to expect the ‘sudden noise’, during the pre-landing patter. Can’t remember the airline, though, because it was a stand-in for Ryanair. (Which in itself could be humourous…) ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [October 29, 2004, 12:14am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/33 "2004-10-29T00:14:29Z")
> [@gotpasswords](#): > > But… You don’t want to stop generating forward thrust just yet, in case you need to TOGA (take off and go around) or otherwise abort the landing, which is why aircraft generally touch down with the engines at or close to full throttle. > > I’m not a pilot, but the landing sequence is generally along the lines of: > > Approach runway at low speed with full flaps (to lower the stall speed) > Flare > Push throtles up \* > Deploy reversers \* > Touch down > Commit to landing > Cut engine power > Apply wheel brakes > > - Something that hasn’t really been touched yet is that in addition to the increased noise simply from running up the thrttle, the reversers add their own noise as they disrupt the exhaust flow from the engines. if you deploy the reversers before touching down, you will stall the plane and it will fall to the ground. This is considered a bad thing, and could be the difference between a good landing and a great landing\* Having watched the thrust reversers deploy on more airplane flights than I can count, I can say for a fact that the main gear is on the ground before the reversers come on/open, and power is applied. \*A good landing being one where you walk away from it. A great landing is when the can use the airplane again ---
**Author:** ![NurseCarmen](https://avatars.discourse-cdn.com/v4/letter/n/5daacb/32.png) [@NurseCarmen](https://boards.straightdope.com/u/NurseCarmen)\ **Post date:** [October 29, 2004, 12:26am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/34 "2004-10-29T00:26:23Z")
> [@don't ask](#): > > 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. You forgot the fluegel valve. And the blinker fluid. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [October 29, 2004, 12:27am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/35 "2004-10-29T00:27:22Z")
> [@Rick](#): > > A good landing being one where you walk away from it. A great landing is when the can use the airplane again I like that, as a motto for air travel in general 🙂 ---
**Author:** ![Pushkin](https://avatars.discourse-cdn.com/v4/letter/p/ee59a6/32.png) [@Pushkin](https://boards.straightdope.com/u/Pushkin)\ **Post date:** [October 29, 2004, 12:28am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/36 "2004-10-29T00:28:58Z")
> [@DougC](#): > > in that atmospheric engines produce thrust not so much by “pushing air out their rear ends” but by “vacuuming air from their front ends”. Isn’t that what the British told anyone who enquired about Frank Whittle’s aircraft when it first flew to put them off figuring out the idea behind it? ---
**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 29, 2004, 4:08am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/37 "2004-10-29T04:08:00Z")
> [@DougC](#): > > Consider this: a jet engine is at least three sections: a compressor, a combustor and an exhaust turbine stage. If the pressure/temperature from the exhaust-section of the engine was ever greater than the pressure from the intake section, then air would start flowing _backwards_ through the engine. I am well aware of this; I mentioned it rather early on, in fact. It is _not_, however, the pressure differential that drives the engine forward, it is the reaction mass expelled from the rear. You could cover the air intake completely and feed pressurized O[sub]2[/sub] from an onboard tank to the compressor instead, and the engine would basically function the same. In fact, this is called a rocket. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 29, 2004, 4:22am UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/38 "2004-10-29T04:22:21Z")
> [@DougC](#): > > My point is that since the forward thrust is really occurring in front of the fans, if you really wanted to stop the fans from generating forward thrust you would be better off stopping air from getting into the fans from the _front_ than you would by doing anything with the air getting blown out the _rear_ I would guess the fan exhaust has a higher _speed_ (because the exhaust nozzle is smaller than the intake duct), and therefore contributes more to the thrust. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [October 29, 2004, 3:14pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/39 "2004-10-29T15:14:09Z")
Most of the thrust in a high-bypass turbofan comes from the fan simply because of the higher volume of air it’s moving, not from its velocity. The Brayton cycle is also suck-squeeze-bang-blow, it just involves moving the air along instead of keeping it in the same place like an Otto-cycle engine. > [@gorillaman](#): > > Would I be right in thinking that at least some designs are highly visible You would indeed. Older low- and medium-bypass engines are usually fitted with the “clamshell” type reverser you see on 737-100’s and -200’s and on DC-9/MD-80 versions. **acsenray** , I assure you the description is no joke. **pushkin** , yes, that’s the legend. **JRDelirious** , you’re right, the squat switch (when it’s working) on the landing gear prevents reverser unlatching unless weight-on-wheels is detected. It’s also shown in the cockpit, at least on some aircraft, by a light labeled “WOW”. ---
**Author:** ![Redsland](https://avatars.discourse-cdn.com/v4/letter/r/9de0a6/32.png) [@Redsland](https://boards.straightdope.com/u/Redsland)\ **Post date:** [October 29, 2004, 3:18pm UTC](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691/40 "2004-10-29T15:18:52Z")
> [@Rick](#): > > The four cycles in an Otto cycle engine are: > Suck > Squeeze > Bang! > Whoosh Around these parts, we give the list as: Suck Squeeze Bang Blow (It kinda sounds like the shot list for an adult film 😉 ) [Previous page](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691.md?page=1) [Next page](https://boards.straightdope.com/t/whats-that-roar-when-a-commercial-airliner-lands/271691.md?page=3)