At 18.10LT on Thursday Apr. 12 emergency agency telephone switch boards started receiving reports of a large bang or explosion that could be heard all over the southern UK.
It took a couple of hours before the reason for the mysterious bang was made public.
As most countries do, the RAF maintains a number of armed fighter aircraft on alert for air sovereignty and security purposes. The pilots are dismounted but are at a heightened state of readiness and can be airborne within minutes.
Since 9/11 this alert status also includes the possibility of reacting to potential threats from terrorist organisations using civilian aircraft to carry out acts of terror by using the aircraft as weapons of mass destruction.
Image credit: Nicola Ruffino
The Quick Reaction Alert (QRA) pilots are managed by the RAF’s National Air Operations Centre (NAOC), a team of 12/13 members including an Air Defence Wing Commander located somewhere West of London in an underground facility.
The team is charged with air policing of the UK airspace and also areas that come under the NATO umbrella, however, it does not monitor the national sky continuously, a task done by RAF’s Control and Reporting Centres (CRCs). CRCs constantly monitor the UK airspace and work in conjunction with the civilian Air Traffic Control based at Swanwick (Hampshire) and Prestwick (Scotland).
What happens usually when a civilian aircraft starts to raise suspicion that it is acting in a manner that is unusual?
It will first be contacted on the international emergency frequency: 121.5 MHz. Pilots are supposed to monitor this frequency at all times, but sometimes pilots use their second radio to listen to weather reports and other more mundane transmissions.
Therefore they miss this initial contact, prompting the initial investigations on the FPL (Flight Plan) filled by the pilot, the planned route and so on.
If further attempts to contact the aircraft fail, the civilian ATC will contact the CRC who in turn contact NAOC who will probably contemplate a tactical response while the problem within the UK airspace will be possibly notified to other NATO countries.
At this point the QRA pilots may be ordered into their cockpits, power on and be ready for immediate start. When engine are started, interceptors can be airborne in around 3 minutes. If the aircraft is still not responding to ATC or the airline on “company frequency” the QRA jets will be scrambled, along with a tanker aircraft.
Once airborne all civilian aircraft will be vectored out of the way of the QRA jets en-route to the target.
Whilst trying to contact the suspicious aircraft, the RAF jets will perform a very wide intercept (out of target pilots visual sight) and approach the target from astern (rear) with transponder switched off the mode C o so as not to alert TCAS (Traffic Collision Avoidance System).
As the jets gets to within visual sight of the target the pilots will “shadow” the plane while performing visual checks to see if there is any visual reason for the aircraft not responding (maybe electrical issues).
If there is nothing obvious, the first jet will approach the target on the left hand side and forward of the cockpit so that the flight crew on the target aircraft cannot fail to miss the jet. The jet will then use the international intercept procedure, including visual follow me signals. Obviously, if the target fails to co-operate then things will be taken to the next level which could mean, after some further attempts to contact the plane and to make it comply to the visual instruction, to shoot it down.
There is a process where all of the above may not take place so quietly sparking an immediate reaction by the QRA cell, and that is if a pilot enters a certain squawk code into the transponder to indicate that the aircraft, has been hijacked.
That’s what happened on Apr. 12, when a hapless helicopter pilot accidently entered the 7500 squawk code that said he had been hijacked, sparking an immediate reaction by the British Air Defence.
At RAF Coningsby the two Eurofighter Typhoon QRA jets were scrambled immediately, call signs 5KG41 and 5KG42 screamed into the sky in full afterburner, and cleared supersonic. Since it is very unusual for combat plane to fly supersonic at low altitudes one of the fighter pilot was heard on the radio asking to confirm the instruction.
When the cleareance was confirmed the interceptors accelerated trough Mach 1.2. The sonic boom was heard by thousands of people who immediately called the police and fire services to report the unusual loud “bang”. Even the British Geological Survery was contacted to see if the UK had suffered an earthquake.
By the time the Typhoons were closing in on their target, the helicopter pilot had realized his mistake and had tuned his squawk to the correct code. A bit too late: the interceptors had already caused some concern throughout UK and their supporting tanker had also been in the air to support them.
Once everything checked out, the event which sent the British media into a frenzy, was all over.
Aviation enthusiasts in the UK noted that the VC-10 tanker was still airborne at 21.45LT a full 3 hours after the intercept and the pair of jets had returned to base at around 21.35LT. The aircraft was also picked up using ADS-B flying circuits off the east coast of the UK over the North Sea. Did they exploit the opportunity to carry out a training mission after being involved in the scramble?
After publishing a post about the Thunderbirds condensation clouds induced by high-G maneuvers and high-AOAs (Angle Of Attack) during and a post about the rehearsals, below you can find some more pictures about the Jesolo Air Extreme 2011 airshow, taken on Jun. 12, 2011.
Yesterday I’ve published some pictures of the Thunderbirds performing a demo flight at Jesolo on Jun. 10 to explain the origin of condensation clouds generated by maneuvering aircraft. Here’s a gallery of the most interesting pictures taken during Jun. 10 and 11 rehearsals of Jesolo Air Extreme 2011.
Even though to the eyes of a spectator a Frecce Tricolori or Thunderbirds display overhead an airfield does not change much from the one which takes place over the coast line of a beach resort, the way display teams or solos fly may differ significantly depending on the environment in which the aerobatic display is executed. The different topographic features of the place where the air show takes place, and the surrounding landscape may, in fact, require the adoption of specific solutions in order to maintain standard distances and to correctly evaluate the separation from the terrain under peculiar light conditions. Familiarisation with the landscape and evaluating the display arena are the purposes of the preparation flight which precedes every display of a display team. In the case of displays flown over land, the terrain usually offers a multitude of fixed references which assist in the perception of speed, travelled airspace and altitude, such as crop lines, fields, roads, railways, and water courses.
Over the water, as at Jesolo, it is necessary to utilise buoys or boats which, besides delineating the display area in respect to a crowd line which is frequently extremely extended, allow the accurate determination of the display line, i.e. the line on the ground that is at least 3 Km long (1,5 Km to the left and right of display crowd centre), which must be perfectly visible from the air and placed at a distance of 230 metres in front of the public. This line constitutes the reference for the pilots for the safe execution of all the manoeuvres.
Although usually free of significant obstacles, displays flown over water can hide several traps as I’ve explained here. In those flown over the sea, the sunlight reflected on the surface may reduce into sun visibility, a phenomenon which also occurs when snow glare is encountered when flying over the mountains.
During rehearsals display teams can fly a modified display to get familiar with the display area and its references. For instance, the diamond formation of the Thunderbirds did not perform the high bomb burst on Jun. 10 while solos repeated some opposing passes while, on Jun. 11, the Frecce Tricolori’s solo did not perform the famous “crazy flight”.
On Jun. 10, 2011, I attended the Thunderbirds’s rehearsal at Jesolo for the Air Extreme 2011 airshow. Pictures I’ve taken show plenty of condesation clouds being generated by the USAF demo team’s F-16s. Even spectators usually think that such clouds surrounding the aircraft represent a visual manifestation of a “sonic boom”, they are the effect of the quick depression on the flight surfaces that brings the water vapour contained in the air at the condensation temperature.
It is a common phenomenon in high-G maneuvers, when the depression on the upper side of the wing increases (as the lift does); it can be observed even at sea level, when the amount of moisture is significant and air temperature is quite hot.
Something quite different are the conic-shape clouds that are generated around aircraft flying at speeds next to the sound’s speed (they are not particularly evident in the pictures taken at Jesolo, though, but you can find many examples here). They are not visual effects of the so called “sonic booms”: more simply, when an aircraft flies at transonic speeds (around Mach 1.0), any of its convess parts (canopy, intakes, etc.) causes a rapid decrease of temperature and pressure with subsequent creation of the cloud. The variation in temperature caused by the perturbation of the airflows is called Prandtl-Glauert Singularity. The particular shape of the cloud associated to the singularity is caused by the perturbation: at that point the airflow can reach supersonic speed and generate a shock wave (that appears when the fluid decelerates and the temperature suddenly raises).
Pictures taken at Axalp rise more or less the same question. People want to know if the condensation clouds surrounding the aircraft represent some kind of visual manifestation of the “sonic boom” or some other phenomenon tied to the flight at transonic speeds.
Actually, what appears in the pictures taken at 2.300 meters of height is nothing more that the effect of the quick depression on the flight surfaces that brings the water vapour contained in the air to the condensation temperature. It is a common phenomenon in high-G manoeuvres, when the depression on the upper side of the wing increases (as the lift does), and that can be observed even at sea level, when the amount of moist is significant and the air temperatures is quite hot.
Something quite different are the conic-shape clouds that are generated around aircraft flying at speeds next to the sound’ speed. They are not visual effects of the so called “sonic booms” (for the last episode in Italy read here: Another supersonic scramble) nor they are the sign of the breaking of the sound barrier: when an aircraft flies at transonic speed (around Mach 1.0), any of its convess parts (canopy, intakes, etc) causes a rapid decrease of the temperature and pressure with subsequent creation of the cloud. The variation in temperature caused by the perturbation of the airflows is called Prandtl-Glauert Singularity. The particular shape of the cloud associated to the singularity is caused by the perturbation: in that point the air flow can reach supersonic speed and generate a shock wave (that appears when the fluid decelerates and the temperature suddenly raises). The shock, due to the quick “jump” from a low pressure / low temperature / supersonic airflow zone to a high pressure / high temperature / subsonic speed zone that is perceived by the human brain as a loud “bang”. Actually, the “sonic boom” has nothing to do with the sound barrier: it can be heard when the aircraft is ALREADY flying at supersonic speed not far from our ears. The sound arrives unexpected because of the speed of the aircraft (that precedes it).
The following video shows a series of pictures the clouds caused by the Prandtl-Glauert Singularity:
The following video give an idea of the sound heard from the ground of a Concorde flying at supersonic speed:
Below, more examples of condensation clouds.
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