Plain Aviation

the life of a planespotter and true avgeek

Month: March 2016

What is a trijet in airliner terms?

private boeing 727 trijet

A private Boeing 727 trijet. Photo by Andrew W. Sieber CC 2.0

As you can probably tell already, the term trijet refers to any commercial jetliner with three engines. Normally they either have all three engines mounted aft of the cabin and near the tail as seen on the Boeing 727, or two mounted on the wings and the third mounted at the tail as seen on the McDonnell Douglas MD-11. One common feature of all trijets is that their third engine is always going to be embedded inside the vertical stabilizer to ensure symmetrical thrust output. While this feature increases the difficulty of design, production, and maintenance, the cost-savings of operating trijets over four-engined airliners is definitely worth it.

Pros and Cons of planes with rear-mounted engines

delta connection bombardier crj900 n691ca rear engines

A Delta Connection Bombardier CRJ-900 with two rear-mounted engines. Photo by Andrew Cohen CC 2.0

Ever noticed that some airliners have a sleek-looking T-tail and engines mounted at the rear? This design is found most commonly on smaller aircraft such as regional and private jets. Some people hate it, while others think it looks sleek and stylish. Now you may be wondering why some aircraft have engines at the back and others don’t. There are actually several reasons why aircraft manufacturers would place engines at the rear on aircraft like the Boeing 717, 727, McDonnell Douglas MD-80, and Bombardier CRJ series to name a few.

Why can’t we use our phones on an airplane?

Airlines have always been very strict on the use of cell phones in-flight due to the belief that they will affect sensitive aircraft systems. Before Airplane Mode was invented and made common, passengers had to shut down their phones entirely for the duration of the flight. But of course, that was also before smartphones became popular and people read newspaper and magazines or talked to their neighbor. Nowadays people are generally allowed to use their electronics only on Airplane Mode and after the aircraft has reached 10,000 feet, although specific regulations vary and

Debris from MH370 found off Mozambique

malaysian airlines 777-200er by christian junker

A Malaysian Airlines 777-200ER similar to the one operating MH370 when it disappeared. Photo by Christian Junker. CC 2.0

For the second time since the disappearance of MH370 in March of 2014, debris from the “crashed” airplane has been found. Last time it was a flaperon (a movable control surface) from the wing found on Reunion Island, this time it’s a piece of the horizontal stabilizer ( found near Mozambique. Experts have agreed that the piece, painted with the words “NO STEP”, is indeed from a Boeing 777, and therefore

What’s that barking noise you hear on the A320?

If you’re an aviation enthusiast like the rest of us or you regularly pay attention to what’s happening during your flight, you may have noticed that the Airbus A320 produces a very audible “barking noise” while on the ground. Most of the time it’s heard prior to or during taxi departing from the airport. Those of you who have noticed it can probably agree with me in that it’s a very distinct noise, most similar to a dog barking but much more mechanical, like a motor failing to start. The noise comes from something called the power transfer unit, or PTU for short. What the PTU does is that it transfers power (hence the name) between the different hydraulic systems so that each individual system has an appropriate amount of pressure. What people normally find strange is that this PTU noise only heard on A320 aircraft, never on the Boeing 737 or Airbus A340. This is because the A320 has a unique hydraulic system that isn’t found elsewhere. The exact technical details of the A320’s hydraulics system is definitely beyond my knowledge and probably beyond your interest, but the A320 has three different hydraulics systems whereas the Boeing 737 only has two, and therefore the A320 needs to transfer pressure between its three systems more frequently than other aircraft and therefore it requires a more powerful PTU. And typically with heavy machinery the more powerful it is the noisier it will be, which is why the PTU on the A320 is audible from inside the cabin. The PTU activates mainly after Engine 2 (starboard) has been started, and since the A320 somtimes taxi’s on one engine to save fuel, the PTU can also be heard during taxi prior to approaching the runway when pilots would start the remaining engine in preparation for takeoff. Next time you’re flying on an A320, be sure to look out for the sound of PTU if you haven’t already, it’s definitely hard to miss and very cool once you know what you’re listening to.

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