18 votes
Accepted

Does European airspace use the metric system?

European Civil Aviation Authorities use the imperial system to depict altitudes and airspace restrictions (feet), speeds (knots) and distances (nautical miles). Some VFR aircraft have speed ...
  • 15.6k
16 votes
Accepted

How is the wind direction and wind speed measured in flight?

Indirectly, as the difference between true air speed and ground speed. Since aircraft cannot measure true air speed, several steps are needed to arrive at the correct wind speed: The pitot tube (big ...
14 votes

Does European airspace use the metric system?

Europe uses imperial for distance (nm), speed (knots) and altitude (feet). Russia and China are the only major countries that use meters for altitude - and it causes all sorts of confusion. Some ...
  • 391
14 votes
Accepted

Why are pitot-static measurements preferred over other velocimetry techniques?

The linked article says that an HWA (hot-wire anenometer) is fragile and requires complex equations to relate the sensed value to the fluid speed as well as correct for inherent, unavoidable ...
10 votes
Accepted

What is the effect of altitude on true airspeed?

If you know GS and local wind speed, the TAS always the same, regardless of altitude. The true airspeed is called "true" because it is exactly how fast you are moving relative to the air. Picture it ...
  • 12.5k
8 votes

How is windspeed scaled in wind-tunnel tests?

In order to achieve the same flow conditions, you need to match several similarity numbers. The most important one expresses the ratio of inertial forces to viscous forces within a fluid and is called ...
8 votes
Accepted

How is the camera positioned for these plane-to-plane Schlieren images of shock waves?

Where is the camera relative to these target aircraft, and what's behind them? Straight from NASA themselves: NASA flew a B-200, outfitted with an updated imaging system, at around 30,000 feet ...
  • 31.7k
8 votes

Are there any companies that use atom interferometers in aircraft?

Probably not. The 2012 paper Advances in Atomic Gyroscopes: A View from Inertial Navigation Applications* states in the abstract that "there are still lots of problems that need to be overcome to ...
  • 5,038
7 votes

Why doesn't the aviation industry use SI units?

You state that "the transition should be much simpler in terms of human factors." Ideally, yes. In practice, no. When mistakes happen, they may well be lethal. The Gimli Glider incident was ...
  • 10.2k
7 votes
Accepted

How is horsepower measured for aviation piston engines?

Like anything else a Dynamometer is used except the engine is bolted directly to it (most likely on a test bench). In a car a rolling road is simply a way to couple the tires to the dyno without ...
  • 97k
6 votes

Does European airspace use the metric system?

As others have mentioned, the basic units to define the airspace are feet and nautical miles. However, some other values are also used when navigating the airspace, and a few are from the SI system. ...
  • 1,677
5 votes

Is there an explanation for the measurement system used in aviation?

To quote from the Wikipedia article on the subject: Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because standard nautical ...
5 votes

How does Kalman filtering work in inertial navigation systems?

A Kalman filter is used on all kinds of IRU's and INS's, even with airspeed information as in an ADIRU. A Kalman filter is just a general method (and a very useful one) for state estimation and ...
  • 6,423
5 votes

Why are pitot-static measurements preferred over other velocimetry techniques?

Hot wire anemometers are great for measuring turbulent fluctuations on a very fine timescale (say kHz). This is really not needed for aircraft speed. The very same wind tunnels that use HWA or LDA (...
5 votes
Accepted

How long should a pitot tube extend from the wing?

That’s going to vary from airframe to airframe based on windtunnel data. Ideally you’re searching for a place where the tube orifice has good access to free stream air that is undisturbed by airflow ...
4 votes
Accepted

How does Kalman filtering work in inertial navigation systems?

First of all you should know that a Kalman filter is a state estimation technique. More than a filter, it is an estimator. Whilst it does filter sensor noise in the process, the most important use is ...
  • 76.3k
4 votes

What is the effect of altitude on true airspeed?

The true airspeed (TAS) can be calculated from the indicated airspeed (IAS), which is derived from the pitot tubes and static ports, as follows: $$ \mathrm{TAS} = \mathrm{IAS} \sqrt{\frac{\rho_0}{\...
  • 49.6k
4 votes

How is the camera positioned for these plane-to-plane Schlieren images of shock waves?

Supplementary answer: As the question mentions this NASA page shows a diagram of traditional Schlieren photography and it's the idea I had in my head. I assumed you'd need a carefully prepared light ...
  • 4,650
4 votes

How do test pilots verify TAS?

The "Brauniger IQ Comp" variometer which is at least two or three decades old now is a rate-of-climb instrument for hang gliders. It is capable of producing a wind direction and wind speed ...
  • 18.4k
4 votes

How do test pilots verify TAS?

On the newest experimental development programs they use what are called "Smart Booms", or Smart Air Data Test Probes, for the test a/c, which is a flight data probe or boom that doesn't ...
  • 111k
3 votes

How is windspeed scaled in wind-tunnel tests?

It really depends on what data you need and the conditions you need it for. Let's say you want to test a scale aircraft model in a wind tunnel in order to predict the lift forces on the full-sized ...
  • 340
3 votes

How does Kalman filtering work in inertial navigation systems?

A Kalman filter reduces noise. If the sensor has a systematic error, a Kalman filter won't help you. Basically, the Kalman filter extrapolates an expected value and compares that to the next ...
3 votes
Accepted

Is there a database of airport ceilometer raw data?

OGIMET looks like it goes back to 2005. My favorite weather site Weather Underground has historical data. It seems like you need to plug in dates one at a time but there are plenty of ways to skim ...
  • 97k
3 votes

Who decides what measurement units are used for altitude, and why is there no standard?

As to feet being the best measurement for height, what utter nonsense! Prior to WW2, all metric countries flew in meters. That changed after WW2 with America dictating what happened on this planet. ...
3 votes

If the altitude of the aircraft is given by the pitot tube, in icing condition with heat on, how is real altitude /speed interpretation done?

The pitot tube measures pressure, not density. The pressure is not affected by the heating of the tube.
  • 76.3k
2 votes
Accepted

Still using knots, nautical miles, inches of Hg. Is that impossible to switch to SI units?

For me metric or imperial is not the question, from my perspective the key is standardization. Having 2 different unit systems has only got drawbacks: Customers: many airline's customers are living ...
1 vote

How to calculate DDM in a ILS dual frequency system?

I don't think you're supposed to just average the two together somehow by themselves. The two signals are designed to be used in different zones and in practice have very different signal strengths at ...
  • 6,423

Only top scored, non community-wiki answers of a minimum length are eligible