Friday, June 28, 2013


Distance Measuring Equipment (DME) what is it and how does it work? Most large aircraft and some smaller aircraft have DME equipment. It seems to be so easy to use, and when it is there, we just tune to a frequency, and it tells us how far away you are from that station.

The simplest explanation is that the DME box is a transceiver that has a computer in it. The transmitter part sends a signal out to a ground station, usually co-located with a VOR. The ground station then will send a signal back to the DME receiver on the aircraft. The time between the signal being sent, and the return signal being received will be computed by the computer, and converted into miles.

The DME signal isn't actually on the frequency that you tune into. The VOR frequencies and the DME frequencies are paired. The pairing is outlined in the AIM, and other documents. A VOR on 111.0 will be paired with a DME on channel 47 (Aircraft Transmits on 1072MHz, Ground transmits on 1009MHz). The channel concept helps us think about the transmit/receive frequencies.

Each DME transceiver sends a unique set of pulses. The ground station sends the same set of pulses back. If their are several aircraft near the same DME ground station, the aircraft only will do timings on the signal with the matching pulses. The aircraft transmitter will listen for quiet time before transmitting to prevent signals colliding. Most ground stations are capable of servicing about 100 unique aircraft at a time.

The distance measured will be straight line. That means, that if an aircraft is flying at about 10000ft straight over the top of the DME ground station, the DME indicator will read 2miles, not 0. The DME system will try to maintain about a quarter mile, per ICAO requirements. The system will include the ground station and the aircraft equipment.

Some of the RNAV and RNP requirements can be met by using two DME systems along with the inertial reference unit (IRU). Approach plates will sometimes be labeled DME/DME/IRU as needed to meet the requirements of the approach.

GPS is making some of the DME capabilities obsolete. Will the FAA begin decommissioning them any time soon? Probably not. Aircraft upgrades are expensive, and the DME systems are quite reliable, and low maintenance. Potentially when all aircraft are equipped with GPS will the FAA consider removing DMEs from the NAS.

Is this helpful?

Saturday, June 22, 2013

ASDI what is it?

When we go to, FlightExplorer or any of the other flight tracking web sites or apps, they have a lot of good information. The information all comes from the FAA, for free! Ever think about the FAA and how they collect that information? How does is all come together? 

All over the country, there are RADAR sensors. These RADAR sensors are scanning the skies 24 hours a day, 7 days a week. The output of the RADAR sensor is sent to a computer, where the range and azimuth data is correlated to the transponder and altitude data. In the drawing below, the black line coming out of the RADAR dish is the interrogation signal, the black line coming back it the "skin paint" reflection signal, and the blue line is the transponder broadcast from the aircraft.
The computers correlate the transponder code to the flight plan, and take the RADAR returns and calculate a speed, altitude and track that the aircraft would be on. The flight plan helps determine where the airplane will be, based on the speed and time since last sample. 

All of the track information for all of the RADAR systems are sent to the FAA command center where they are made available for display in the various FAA systems that need the information (IE URET, TRACONs, ERAM, TFMA, etc).

One of those system that get the FAA RADAR data is the Aircraft Situation Display to Industry (ASDI). The ASDI data is availble to the airlines and other organizations in the aviation industry. The information includes flight plans, position reports, departure, and cancel flight plans. Using this information sites like FlightAware can present aircraft on maps for the general public. 
The ASDI data only contains non-blocked RADAR and flight plan data for aircraft with IFR flight plans in the US and some of Europe. There is an option available to private aircraft allowing them to block the ASDI data for competitive reasons (IE the president of ATT doesn't want the Verizon corporation to know about some special meetings with a partner or something).  

There are about 4 different kind of feeds of ASDI data. There is the internal FAA feed, the need to know real-time feed, the need to know real-time with European data, and a delayed feed. The real-time feeds are for the airlines and such to use for business reasons. The delayed feed is for the web sites visited by the general public. The delay is like 5 minutes, so it is good enough for people to know when to show up at the airport to get their loved one. 

There is a bit of information that can be derived from the ASDI data. Looking at the ASDI data, someone can determine which airports are taking delays with many aircraft holding. Other things can include looking at the projected track, and weather data to see when it might be best to re-route an aircraft because it is heading toward some convective activity. Airport operators can use the data to count operations relative to other airports, to help improve service. 

The ASDI data feed contains a lot of data. The position reports will be updates of aircraft positions every time the computers are aware of a position update (IE every 12 seconds for enroute RADAR, 5 seconds for TRACON, or 1 second for ADS-B). Typically Monday through Friday in the US there will be 3000-5000 aircraft in the air from 6am to 6pm. 

The FAA has many other services similar to ASDI. Most of it isn't as useful. ASDI is a great resource. 

What do you think.

Friday, June 14, 2013

1500 hours or Nothing

 We seem to be at a weird crossroad. Congress is trying to mandate that the first officers, if they are in the cockpit must have 1500 hours for safety reasons. They are also being pushed to keep both pilots out of the cockpit, and leave them on the ground.

The UAV or drone folks want to keep the people out of the cockpit, while the safety people want more hours for the folks in the cockpit. I kind of get it, I guess, pilots are highly paid people, and technology is getting better, such that UAVs are pretty reliable. If only drones are in the air, then they should all cooperate, and everyone should be happy.

Well, how would you feel about a cockpit with no one in it while you were being whisked on your vacation in the Bahamas? There is someone on the ground paying attention to your airplane, should anything be out of the ordinary. They are paying attention to six or seven other flights as well, heck aircraft on autopilot re all pretty reliable.If the autopilot notices anything unusual, the pilot on the ground will control the aircraft to a landing.

Ice seems to be a common failure mode for recent passenger aircraft crashes. The Colgan 3407 had a captain that had switched aircraft types, and may have been confused about proper action with ice. The Air France 447 crash had ice that caused the autopilots to give up, and ask the less experienced co-pilots to fly the airplane. It is probably good that the FAA mandate more experience to crews, to insure that should something out of the ordinary happen, they will be able to take the proper action.

How much experience should someone on the ground have, if they are needed? Based on recent incidents, they ought to have lots of experience. They will not be dealing with "normal" flights, only abnormal situations. Maybe they will trying to get an ice laden commuter to a safe place at an airport, or a larger transport aircraft through a massive thunderstorm with no reliable airspeed indication. Either way, they will need all the feedback they can get to know what the situation is.

Airplanes are built on many systems. The pilots job is to be able to manage all these system in all situations. Sometimes the indications are providing questionable feedback, and correlating different dis-separate systems can yield hints to the true trouble. The human brain is still better at tasks where the data is really fuzzy.

There are arguments, should pilots be trained in full motion simulators, or are fixed simulators good enough. Well there certainly is a good bit of seat of the pants information that is available in a full motion simulator, but for many situations, the basic procedure trainer will get the normal flying situations covered.

 Should the remote pilot be in a full motion cockpit to help fly this broken airplane? I don't think anyone is considering that. Mostly the remote pilots are going to be expected to fly from a desk in an office somewhere.  Typically it will be a cockpit looking desk, but the chair will probably be on wheels, and just a couple computer displays will be in front of the pilot.

Depending on how bad the broken airplane is broken, it may not be able to provide any feedback. Maybe sensors have gone bad, and that is why the autopilot has given up would be the primary reason the remote motion cockpit will not work. Sometimes the computers in the aircraft don't work, and the remote pilot is going to rely on backups to backups.

Datalinks go bad. We are all used to always on internet, but how often does your internet go out? Your home internet isn't moving, so it should be very reliable. If you have satellite TV when it rains, what happens? Well, imagine flinging through the sky at 40000 feet, in a thunderstorm, 1500 miles from any land, how reliable will the communications link be there? Satellite is pretty reliable, especially in the rain? How about ground links, 1500 miles from the nearest based station, VHF and above won't cut it, and HF is too slow. So the autopilot should be 100% reliable, after getting struck by lightning 2 or 3 times? maybe.

Look I love technology, but I like to relax on my vacations. I don't mind paying a few bucks for the pilot to be sitting up in the front of the airplane. He has some skin in the game. If he messes up, he gets as hurt as me. A guy sitting in an office, might not think things are so important.

What do you think?

Monday, June 10, 2013

Installing Equipment

A few years ago, I bought my wife a Toyota Sienna. It was mostly top of the line, with SatNav, CD Changer, Bluetooth, DVD player and all the bells and whistles one might want. Well, time marched on, and now the SatNav looks old. I don't use CDs in the car, I use Pandora, or MP3s that I have. The Bluetooth doesn't do A2DP, so I can't play music from my cell phone while in the car unless I plug in. The DVD player only plays DVDs, and not Blueray. It isn't even that old, but it seems my cell phone has passed it by.

Looking back to about that same time for avionics, and things have updated there as well. The GNS 430 and 530 were the top of the line radios back then. Today, we kind of admire the aircraft with those radios, but would rather have the G1000 or something newer, given a choice. How about today, well Garmin hasn't been standing still, and are ready to offer the latest better things.

What should someone do who wants the best latest avionics? I believe we are on the edge of the future. What if the (Attitude Heading Reference System) AHRS and Engine Monitor were mounted in the aircraft, but the display you could update? That would allow some lower cost better newer looking panels.

What if we could use our tablets as the display, and update the software and data over the air (OTA)?  Well today people are doing just that. Sporty's sells AHRS's that you can get today that will display the results on your tablet. Today they are only "backup", because they aren't bolted into the aircraft. I'd wager, people with these systems are using them as the primary navigation display, and relying on the less functional panel equipment as the backup.

Say you did a proper bit of engineering to mount the gyros, and added some pitot/static data. Send the data over USB or WiFi to a tablet that would also be mounted in the panel. What is the difference than a panel mount system? It will work for experimentals, and maybe part 91 operations. TSO equipment is required for Part 135, so it won't work on aircraft for hire.

It can be done, or at least something to think about. How far would someone have to go to get one of the tablet systems TSOd. I am sure there are documents that the FAA and others produce that would tell me. That will be my research of the next couple weeks.

Keep reading.