Planes have windshield wipers
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Question: Thank you for your column. I have been forwarding your articles to many of my friends who now see airline flight as comfortable and commonplace as riding in a bus or a taxi. On a recent flight into O'Hare, we descended for several minutes through thick cloud cover. This got me wondering — how do you clear the cockpit windshield in the case of heavy rain or snow? And why doesn't the water vapor from the cloud cover condense to the passenger windows as the plane flies through it? I do sometimes see ice or moisture outside the passenger windows but it usually accompanies rain or snow.
— Gerald Lack, Chicago
Answer: Thanks very much and it has been my goal all along to help passengers fly more knowledgably and, as a result, more comfortably.
To answer your question, planes have windshield wipers. They have multiple settings for different speeds just like the ones in your car. Unlike the ones in your car, the two wipers operate independently, giving each pilot control over his or her windshield. There are also window defoggers in the cockpit.
Some planes have specially coated windshields to repel water. Others utilize "rain repellant" for the windshields in very, very heavy rain. If a plane uses rain repellant, normally there is a button located next to the wiper knob on the overhead panel just above each pilot's head. A single push of the button delivers a measured amount to the windshield. Rain repellant is thick and gooey, so if it's used on a windshield that isn't wet enough, it can be somewhat messy. However, when used correctly it's like "turning off" the rain. The last airplane I flew that had rain repellant was the B-727 and I only had occasion to use it once. It certainly did the job. It's interesting to note that neither windshield wipers nor rain repellant is used during cruise flight. They're used primarily on final approach if rain is obscuring the view of the runway.
In this photo, used with permission and showing the overhead panel of a Lufthansa A-340, look in the lower left-hand panel area to find the windshield wiper knob with the rain repellant button just to the left.
Passenger windows have 3 layers (Related column: When passengers get punchy). The middle layer has a pinhole in it which allows that outer layer to be pressurized and prevents fogging. The outside of these windows is not conducive to the formation of condensation on them, much as the side windows of your car remain relatively clear even when you drive through rain and fog.
Question: Thanks for taking to time to enlighten all of us. I very much enjoy reading your columns. As a business traveler and an aviation enthusiast, I pay close attention to the mechanical systems of each jet I fly. I was just wondering, how in the world are jets able to invoke "reverse thrust" so quickly upon landing? Do the jet turbines actually stop and start turning the opposite direction or is there a blade change or something to accomplish this feat?
— Bob Workman, McAlester, Okla.
Answer: I'm so glad you enjoy my columns.
The engines don't literally reverse. Rather, thrust is deflected outward and/or forward. There are "reverse levers" the pilot simply raises to get into reverse thrust, and lowers to get out of reverse. The reverse thrust levers are physically attached to the thrust levers.
Just prior to landing, the thrust levers are normally at idle to avoid excess airspeed, which can increase the tendency for the airplane to "float" down the runway before touching down. Once on the ground, the pilot flying simply lifts the reverse levers to achieve reverse and can modulate the power just as when using forward thrust. You can see the reverse levers just forward and attached to the thrust levers at the following link. This photo shows the cockpit of a Deutsche British Airways B-737 on the ground in Munich. You can see the two little white knobs just ahead of the thrust levers and, although difficult to see here, those white knobs are attached to the thrust levers. When ready for reverse thrust, the pilot will move his or her hand forward so the wrist is on the thrust levers, leaving the hand available to manipulate the reversers.That square white lever to the right of the thrust levers is the flap handle, and the taller white lever to the left of the thrust levers is the speed brake lever. (Thanks to Florian Joachim, who works in Berlin for a German air carrier, for giving us permission to use this image.)
Some airplanes were designed to be able to use reverse thrust in the air to help facilitate descent, such as the DC-8, but the majority of airplanes have a mechanical stop to prevent inadvertent reverse thrust in flight or whenever forward thrust is being used. Some airplanes don't have reverse thrust capability, like the Fokker F-28, which is somewhat similar to a DC-9. Reverse thrust enhances stopping capability and helps prevent wear and tear on the brakes, but it is not actually required and not considered when the manufacturer calculates the stopping distance capability of the airplane.
There are different designs of reverse thrust. Older aircraft used "clam-shell" or "bucket" reversers. In these designs, panels come out behind the engine and deflect the air, much as air from your mouth would be deflected if you blew while holding your hand up in front of your face. You can see the "clamshells" fully deployed in this photo (taken in Buenos Aires by Spanish pilot/photographer Ander Aguirre).
Another type of reverser is known as the "cascade" type. In this design, the engine casing slides open to let fan air deflect outward and forward. You'll find this type of reverser on newer jets using high bypass ratio engines. The cascade system avoids the extra weight of the older clamshell-type reversers and is considered more economical and more practical for these larger engines. Here is a photo of a military C-17 in reverse with the engine casing open (this photo is used courtesy of www.aerospaceweb.org).
On propeller-driven planes with reverse thrust, the pitch of the blades is simply changed to a negative angle, which directs airflow forward.
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Meryl Getline is a pilot, author of the award-winning book The World at My Feet and a keynote speaker for corporate and other gatherings. She also publishes her own websites, fromthecockpit.com and flyingfearless.com. All opinions expressed in this column are exclusively those of Capt. Meryl Getline. If you have a question, send it to her at firstname.lastname@example.org, acknowledging she may use it in a future column.