How does an aircraft just disappear?!?

This is the question I have heard most from the public in relation to the disappearance of MH370. It is an embarrassing situation but not all that uncommon during normal operations. I will attempt to answer the question in terms that the media has somewhat confused.

Primary Radar

Most people are aware of the basic principle of radar. The first generation of radar involves the emission of radio waves that are sent out by a radar dish (typically on the ground), are reflected of an aircraft or object and ‘bounced’ back to the same radar dish. The electronics translates this radar return as a distance and bearing from the radar and can also calculate speed based on that information. This is displayed pictorially on Air Traffic Control radar screens.

The main short fall of this technology is that it is truly raw positional data and cannot depict additional data such as the flight number (callsign), and altitude (military radars can depict altitude). Additionally it is limited in range.

Secondary Radar

Also known as Secondary Surveillance Radar (SSR) is second generation tracking technology is aptly referred to as secondary radar. This technique builds on the idea of primary radar. A second radio wave leaves the radar station and ‘interrogates’ a transponder located on the aircraft. The aircraft, in turn, replies and transmits it’s altitude and a code which is assigned to it by air traffic control. The ground station uses the time taken for the radio wave to bounce and the aircraft’s reply to determine the position and speed of the aircraft and also receives the altitude information and code. The radar electronics uses the code to correlate the aircraft data to the flight plan filed by the flight crew. So now the air traffic controller can now see the position, speed, altitude, flight plan details and other information about the flight.

Additional advantages include increased reliability (fewer false radar returns) and better reliability. Also, aircraft can select emergency transponder codes to communicate non-verbally to air traffic control during radio failures or other events, including unlawful interference.

The useful operating range of both primary and secondary radar varies with the height of the radar station above sea level and the aircraft’s altitude since both systems require line of sight. This means at cruising altitude of large passenger transport aircraft, a range of around 150 nautical miles.

OK, so what’s this ADS-B thing I’ve heard about?

Radar installations are expensive to set up and maintain and, as I’ve mentioned, have some limitations in range and use.

ADS-B is a relatively new technology and is considered state-of-the-art for tracking aircraft. The information is wholly derived on board the aircraft using GPS. That is to say; the aircraft knows it’s own GPS position, altitude, heading and flight number. It transmits this information twice per second via the transponder (a modern type that is also compatible with secondary radar) to a ground station.

The cost of installing an ADS-B receiver is minimal when compared to radar installations. This means that you could place a receiver either side of a mountain range and monitor aircraft on both sides of the mountain that would ordinarily be blind to one or both of an equivalent radar station.

So there are fewer errors associated and it’s cheaper to install ADS-B compared with radar.

Is that the same as ACARS?

In short, no.

Primary and secondary radar along with ADS-B are what Air Traffic Control use to track aircraft. ACARS is used for the airline to communicate with aircraft in flight and vice versa.

ACARS is essentially a text and information service. Flight crew use it to retrieve up-to-date weather information, pre-flight information and other uses.

The aircraft uses the information to regularly transmit aircraft engineering parameters to maintenance controller so that fixes/parts to problems in flight can be addressed while the aircraft is still inflight and the flight crew are doing other things.

Got it! But how did air traffic control lose them?

All of the data from the systems above that I have mentioned is combined into various air traffic controllers screens as a target, with flight telemetry along side it. The flight appears on different radar (surveillance) screens during it’s journey. Various countries administer their own airspace. Some of the raw data is shared, some is not.

The raw data is turned into a situation display which is what the air traffic controller ultimately sees.

There are times when aircraft fly outside of the range of the ground equipment that detect them. In such cases, flights are procedurally separated. This means that the flight is tracked by the methods above until it is out of range. Up until that point, a highly accurate position is known to the controller. Once out of range, the aircraft will call the controller on the radio at regular, published intervals with the name of their position, the time at that point, their altitude and their estimated time of arrival at the next reporting point.

The controller will then enter this information, and their situation display will update with a ‘calculated’ position. This sound less exact than it is, and the required separation standard for aircraft in flight is increased to account for margins of error.

Ok. But what about MH370?

Good question.

From media reports, it sounds as though everything was going well until the crew reported their position at IGARI. From here it ‘disappeared’ from the controllers’ screens.

Such occurrences are not infrequent. Transponders fail, GPS may be unavailable to the aircraft for some minutes, atmospheric anomalies interfere. However the reason so much speculation has gone straight to fowl play is because B777 sized aircraft carry two transponders, just in case one fails.

For the aircraft to stop transmitting, and the crew not respond to calls by radio to verify any problem is where things start to get strange.

There are three logical explanations for everything going quietly so quickly:

  1. A total, rapid electrical failure. This would need to be a profound failure as there are four sources of elctical power on the B777 and this is distributed to provide redundancy for navigation and communication radios. In other words, a fire or explosion would need to occur.
  2. An in-flight break up. A rapid destruction of the aircraft from some kind of massive, rapid structual failure. These are very rare in smooth air and this type of aircraft. Also the theory that the aircraft continued flying discredits this idea – but I’m just looking at why it went of the radar in this post
  3. Deliberately turning off the transponder. Usually one transponder operates at a time but both can be selected to Standby/OFF from the flight deck and this is a very simple process. I will go on to say only that if this is what happened, whoever was in the cockpit at the time DID NOT WANT TO BE TRACKED.

 What’s all this about the aircraft turning back toward KL?

My interpretation from media reports is that the air traffic controllers at the time of the disappearance did not have ready access to military radars and the aircraft was outside the range of any civilian primary radar facility.

When an aircraft fail communication checks, it may take hours to inform the people that need to be informed and for the correct questions to be answered. In this case, it took many hours to either start or to complete, the playback of all the recorded radar information for the hour before and the multiple hours after the aircraft went missing.

Depending on various factors, any primary radar information that is now available, may not allow for positive identification (it’s basically just a dot on a screen) and various black spots on its tracking further hamper efforts to track it successfully to a crash site.

It is not my intention, in this post, to speculate on the various theories behind the disappearance. Merely to try and clear up how it is that air traffic control could ‘lose it’.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s