76 votes
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What is this hatch above the cockpit of a 747?

It's an emergency escape hatch. There are inertial reels in the cockpit ceiling adjacent to the hatch to allow the cockpit crew to exit the airplane. See this link for an account of their use escaping ...
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  • 38.7k
67 votes
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Can experts identify the model using this part (FOD prevention purposes)?

This is a Military Standard fuel cap, part number MS29525-1. Here is the DOD page for the products: MS29525-1. It is most likely from a military aircraft or some kind of private ex-military warbird. ...
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  • 35.2k
61 votes
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What is the offset in a seaplane's hull?

That's called the step. Without it, you'd have to fight against the buoyancy of the rear end of the hull when you rotate for takeoff. However, a seaplane float or hull must be designed to permit the ...
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  • 10.1k
57 votes

Is this one of the engines from the 9/11 aircraft?

They are definitely not jet engines. They look more like electric motors or generators or blower units or something.
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  • 107k
51 votes
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What is this object in the cockpit of an ultralight?

It's an airspeed indicator for ultralights. The pitot inlet is at the bottom and the pitot air pushes a little red plastic disc up and down on a central rod, with a calibrated clearance between the ...
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51 votes
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What's that in front of the overhead panel?

That's where the analogue/backup compass is stowed. The compartment can be opened downward if you ever need to use the compass, as seen in this image:
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47 votes
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What is this on top of the Air Force One?

The aircraft is the Boeing E-4B Advanced Airborne Command Post 'Nightwatch', which serves as the survivable mobile command post for the US National Command Authority. The aircraft are based on the ...
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47 votes
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What is the strange object on 737-800 exterior, on the aft right side?

It's the air inlet for the Auxiliary Power Unit (APU). Slide 18 here has an image labelled "APU Air Inlet door with Vortex Generator". The inset image on the slide looks almost exactly like your ...
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46 votes

Is this one of the engines from the 9/11 aircraft?

These are very large electric motors, used in HVAC plants or water distribution, probably. Or possibly main generators out of a diesel gen-set. They are very dense and tough by design, though ...
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46 votes
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What is the metal bit in the front of this propeller spinner?

It is a fitting for a Hucks Starter. Photos from Vintage Wings Canada
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45 votes

What is this hole at the front of the engine pylon on a Boeing 707?

This is because those engines have turbocompressors which are used to supply cabin air. The engine bleed air drives the turbine of the turbocompressor, which in turn would drive a compressor which ...
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  • 98.7k
44 votes
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What is that device on an Su-27 pilot's helmet?

That is the mounting point for night vision goggles. The following image shows a helmet with NVG mounted. Helmet mounted NVG; image from lemt.by The USAF has helmets with different mounting and some ...
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  • 98.7k
43 votes

What does this black triangle placard on board an A320 denote?

If there is a problem on-board and the crew is unsure about the condition of the aircraft, the triangle indicates what window should be used to inspect flaps, slats or engines or look out for icing. ...
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43 votes
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Which aircraft had such a luxurious-looking navigator's station?

It's a Boeing 314 flying boat. From the late 30s (look at the two rectangular windows behind the cockpit). The last dedicated navigators on long range flights had started to disappear by the late 60s ...
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42 votes
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What are these Lights on the Center Column between Cockpit Windows on the A320?

These are not lights, they are the Eye Reference Indicators. They are used to find the correct seating position. You adjust your seat such that the white ball is exactly hidden behind the red ball on ...
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42 votes
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Why do propeller driven planes have intakes/inlets, such as this one on the P-51 Mustang?

Propellers are driven by engines, and engines burn fuel, and burning requires oxygen from the air. The intake supplies air to the engine. Burning creates heat, and air intakes also direct cooler air ...
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  • 50.3k
41 votes
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What are these five indicators at the front of the 737 passenger cabin?

This is the forward Master Call Light Panel installed in the ceiling of the cabin. A second one is located aft. These panels exist on all large aircraft for the cabin crew members to be alerted ...
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40 votes
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What are those hatches on the engines of a B-2 that are open during takeoff?

These are secondary intake doors which allow more air to reach the engines. They are opened by the engine control computer when the main intakes are not providing enough air for the power setting of ...
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40 votes
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What is this gigantic dish at Ben Gurion airport?

It is just a design feature, and serves no technical purpose, beyond filtering some light and providing a waterfall feature. Safdie designed the roof of the rotunda as an inverted dome, an ...
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  • 25.8k
38 votes
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What are these "pimples" on the jet engine for?

They are the roots of the variable stator vanes that control the direction of the airstream as it enters each compressor stage to manage and optimize the angle of attack of the compressor blades in ...
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  • 107k
38 votes
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What are those bumps on top of the Antonov-225?

According to the Airliners.net forum, they are fairings that cover hardpoints used for attaching outsize loads. Outsize loads that cannot fit into the capacious cabin (including Buran and Energiya ...
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  • 1,052
38 votes
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What is this strut in biplane rigging, attached only to wires, not bearing any load?

It's just a stick set between the flying and landing wires (the "wires" are actually solid stainless steel rods with an almond cross section for streamlining - they are VERY expensive). ...
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  • 107k
37 votes

Why do most ARFF trucks tend to be so angular?

While the upper portion design enables better visibility, the lower part design is also purposeful: ARFF needs to be at least somewhat offroadable, as the plane in distress may not have stopped on ...
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  • 21.5k
36 votes
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What's this black rectangle painted on the side of a V-1 flying bomb?

A "Pallung" is a piece of wood (or something similar) that prevents ships (or something similar) from falling over when they are on land. "abstellen" means to put something down (...
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  • 496
36 votes
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What is this cylinder covered in threaded studs and what is it for?

It's the inner liner of a heat muff, either for cabin or carb heat (you can see the engine mount tubes right next to it and that's exactly where I'd expect to see it, and the black pucks behind are ...
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