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I met someone the other day who owns a small plane and he was going to fly from Cambridge to the Isle of Wight. I asked him whether he had to file a flight plan and he said no.

Just wondering therefore how such collisions are prevented?

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Most air collisions in the uk seem to between military aircraft. – RedGrittyBrick Aug 21 '14 at 13:58
@RedGrittyBrick formation flying is always dangerous, and a lot of the accidents involved trainer aircraft, so probably were formation flying training... – jwenting Aug 21 '14 at 14:00

6 Answers 6

Small airplanes will be under VFR (Visual Flight Rules) which entails that the pilot looks out the window and is responsible for staying away from other aircraft and staying out of the clouds (the fog banks of the sky).

If you see another plane in your path then there are standard rules to avoid him see this question.

They can fly under IFR (Instrument Flight Rules) but that requires that Air traffic control monitors the airplane and steers it away from other planes.

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thanks. I'm quite surprised that it relies on pilots being vigilant... for example what if one plane is descending and another climbing... it's got to be difficult always to keep a visual watch in 3 dimensions... presumably there haven't been many collisions otherwise this system would have been called into question... – mike rodent Aug 21 '14 at 13:12
@mikerodent The pilot must always be vigilant – ratchet freak Aug 21 '14 at 13:24
@mikerodent There are quite a number of accidents and incidents involving small airplanes. Not only mid air collision but also cases of small aircraft straying unintentionally into controlled airspace. There is work going on to make it cost effective to equip small aircraft with ADS-B OUT and ADS-B IN so that an extra early warning for nearby traffic is possible. – DeltaLima Aug 21 '14 at 13:40
@mikerodent Also bear in mind that there are very few planes in the air and, also, that it's much harder to hit something in three dimensions than it is in two. For example, if you were to drive from Oxford to Dover while your friend was driving from Cambridge to the Isle of Wight, your paths would certainly cross (ignoring the possibility of bridges) and you would crash if you both reached the crossing point at the same time. However, if you both flew, your tracks over the ground would have to cross but you might fly over that point at different altitudes as well as at different times. – David Richerby Aug 21 '14 at 13:42
@DavidRicherby yes, that thought had occurred to me... and ultimately engineers deal in probabilities (e.g. that a wing won't fall off an A380)... but the VFR system described is, it seems to me, clearly sensitive/vulnerable to the number of people owning and using small aircraft, and to the restriction of usable areas if ever controlled areas are extended. Particularly in proximity to the runways where small aircraft take off and land. – mike rodent Aug 21 '14 at 13:46

Glider student here.

There is no electronic or administrative system which can replace pilot vigilance. In the US (I am not familiar with the regulations anywhere else), the end responsibility to avoid other aircraft ALWAYS falls with the pilot in command, even when given clearance/directions from ATC.

There are systems to help though. Popular with glider pilots (especially in Europe, but increasingly in the US) there is a system called FLARM which broadcasts GPS and baromatric data. Other aircraft with FLARM units within range receive this signal and selectively warn the pilot of collision hazards. FLARM has a number of shortcomings which pilots must be mindful of; eg: aircraft without a FLARM unit are not tracked, it has limited range so it isn't very helpful for distant or fast-moving aircraft.

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According to Wikipedia: "In the UK the telecom authority and Civil Aviation Authority confirmed in 2007 that FLARM was acceptable and so it is also being adopted there." – RedGrittyBrick Aug 21 '14 at 19:56
+1 for being a glider pilot :) – rmayer06 Aug 19 at 12:04

Under Visual Flight Rules (VFR) "See and Avoid" is used. For every 9 seconds spent looking out of the cockpit scanning the sky for other aircraft, the pilot spends 1 second inside the cockpit checking instruments etc.

An important part of this though is that if the aircraft is equipped with radio then the pilot should talk to an air traffic service, even a procedural service like London Information is useful, because a) the information service will tell the pilot of other known movements in his area and b) by giving out PACER (position, altitude, conditions, estimate request) reports, other VFR aircraft will be able to listen and if necessary change their own altitude in advance if there is any doubt about seeing the other aircraft in time.

Not all VFR aircraft have radios though. There are additional rules like not flying too close to clouds. I believe one reason is that you don't want another aircraft to appear out of a cloud just feet away. Ultimately though, its the MK1 eyeball and the owner of that eyeballs responsibility not to run into other aircraft.

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+1 the importance of radio communications for seeing other aircraft in time. – usernumber Aug 19 at 13:35
Yes its not really see and avoid, its listen and avoid, as in, listen and if you can't see an aircraft that is in roughly your location, make sure you are at least 500 feet above or below their reported altitude. This also gives you a direction to look in - up or down. – Philip Johnson Aug 19 at 14:54

Whilst there isn't really a good all round on board system for GA aircraft in the uk. It's good practice to get a Lower Airspace Radar Service from a nearby unit if available.

The level of service depends on how busy the unit is, but generally you can get some form of deconfliction service which provides traffic information and avoidance steers.

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To add - you don't need to be IFR to get a LARS service. – vectorVictor Aug 31 '14 at 7:27

As has already been mentioned, the primary method of avoiding other aircraft when flying VFR is to simply look out the window and don't fly into another aircraft. However, pilots in the same area are also usually in contact with each other by radio on a frequency that is determined by where the aircraft are operating. Pilots advise each other of their intentions by radio, which allows other pilots to know where they are and what they're planning to do.

The specific case of around airports was mentioned in a comment earlier. This is indeed the area where pilots have to be most vigilant in avoiding other traffic (because there's more of it around airports.) However, for uncontrolled airfields, there are specific, standard traffic patterns that pilots are supposed to fly when approaching or departing from a given runway. While pilots should always be vigilant in monitoring every direction, following this pattern allows pilots to know where to expect other aircraft to be around the airport and monitor those areas most closely. And, again, pilots approaching or departing from an airfield should advise other nearby pilots of their intentions via radio.

Of course, at controlled airfields, pilots are required to maintain radio contact with the controllers and follow their instructions so long as it's safe to do so.

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I am also a glider pilot. The Soaring Society of America (SSA) recently published a rather lengthy letter on collision avoidance, particularly concerning gliders and powered aircraft, in response to the FAA Advance Notice of Proposed Rulemaking.

First and foremost, midair collisions in general are unlikely, due to the vast amount of space available to maneuver vs the relatively small size of (even large) aircraft. That being said, due to conventions and limitations near airports and air routes, along with the sheer number of aircraft, collision avoidance does become an important concern.

For gliders, collision avoidance is particularly important because they tend to be restricted in geographic range due to atmospheric or geographic features. The SSA letter particularly references how this is an issue around the space between Las Vegas and Minden, due to the ridge and wave lift available in that area. I have similar experience flying in the ridge due east of the Washington, DC metropolitan area, which also has a heavy amount of commercial jet traffic inbound to the region's three major airports.

Here are some snippets from the SSA's letter which I believe are relevant:

SSA agrees that expanded use of available forms of anti-collision technology in flying machines of all types – not just in gliders – obviously would make flying safer. Thus, a large number of US glider pilots have voluntarily added to their cockpits either transponders (which broadcast the position of a glider to ATC and TCAS equipped traffic) or an alternative, effective, lower-cost technology called FLARM (which, in addition to broadcasting location information, also provides collision alert data in the cockpit of gliders, much as TCAS does in the cockpit of larger, more expensive aircraft.) FLARM has been widely accepted worldwide and has the support of the European Aviation Safety Agency (EASA.) It is so successful and effective that the use of FLARM (rather than transponders) is now mandated for gliders flying in French airspace.

According to SSA's research, FLARM is more effective than transponders, because transponders require active involvement from the FAA controllers, and controllers typically clear VFR aircraft (which includes gliders) from their screens.

Transponders are not true collision avoidance devices in that they do not provide any direct information to the pilot. They may be useful to other pilots who are equipped with a TCAS (or similar) device, but because of very high costs those devices are not in widespread use except by the highest end of the aviation community. As an additional issue, when air traffic controllers have too many targets on their displays, they may suppress VFR traffic, which would include transponder equipped gliders. And as a matter of policy, ATC does not necessarily give IFR traffic advisories on uncontrolled VFR traffic in its vicinity even if that traffic is transponder equipped. (Gliders rarely operate under positive ATC control and are therefore generally “uncontrolled” even when operating a transponder). Aircraft that fly VFR only and are not in contact with ATC derive essentially no direct safety benefit from equipping with a transponder; doing so only assists ATC in applicable areas and the high end users of the NAS that are equipped with expensive collision avoidance devices like TCAS; or in the case of gliders, with PowerFlarm.


Each FLARM device determines its position and altitude with a highly sensitive state of the art GPS receiver. Based on speed, acceleration, heading, track, turn radius, wind, altitude, vertical speed, configured aircraft type, and other parameters, a very precise projected flight path can be calculated. The flight path is encoded and sent over an encrypted radio channel to all nearby aircraft at least once per second.

At the same time, the FLARM device receives the same encoded flight path from all surrounding aircraft. Using a combination of own and received flight paths, an intelligent motion prediction algorithm calculates a collision risk for each received aircraft based on an integrated risk model. The FLARM device communicates this, together with the direction and altitude difference to the intruding aircraft, to the connected FLARM display. The pilots are then given visual and aural warnings and can take resolutive action.

Read more from the SSA Letter

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