Monthly Archives: December 2006

The "break"

 

This is not a composite picture showing the same aircraft during a turn. It is a photo taken in 2004 at Grazzanise airbase and depicting the first section of the Frecce Tricolori performing the “break” over the runway. The “break” is a common maneuver in military aviation that enables aircraft belonging to the same formation splitting and getting the mutual separation needed to land one by one.  Frecce, as well as Red Arrows, make very aggressive and spectacular low level breaks, but the “break” is simply a part of a common VFR / VMC procedure that can be performed in different ways, depending on the flying characteristics of the aircraft, on aircraft load and weight, on the airport location, on the purpose of the mission, etc.  For instance, the break is also the standard entry to the landing pattern used by conventional aircraft on the carriers. 

As said, the break is just a step of the approach procedure.  Generally, during a VFR recovery, the approaching aircraft has first to overfly an Initial Point, that is an easily recognizable site on the ground located a few miles from the airport (sometimes on the extended centerline of the runway, sometimes displaced to the right or left of it). From the IP the aircraft heads directly towards the threshold of the runway that is overflown at an altitude dependend on the airport elevation. Overhead the runway, each pilot performs the “break” a few seconds after the preceding aircraft, banking violently to the right or to the left of the runway centerline (in accordance with the airport procedures) in order to reduce speed and to enter the downwind leg of the visual pattern, that is a course parallel to the active runway laterally displaced by the field by 1 or 2 miles. As the aicraft slows to the approach speed the pilot configure his fighter for landing, selecting the correct flap position and lowering the landing gear. Beyond a point located across the touchdown point the pilot begins a descending 180° turn towards the runway (the so-called “base turn”) to intercept the final approach course. Flying characteristics of the aircraft affect the length of the downwind leg and final approach distance. The F-104 made very flat approaches performing downwind legs longer than those made by more agile MB.339 or F-16. 

Axalp 2006 report (in English)

In the age of Google Earth and satellite pictures freely available on the Internet, hiding military installations or denying their existence is at least anachronistic.

“Confusing and camouflaging”, favoring the osmosis of the base with the surroundings urban conglomerate, it is much more effective than “hiding” and this is the philosophy that seems to have inspired one of the most atypical airports of Europe: Meiringen, in Switzerland.

Located at an elevation of 570 meters between the Alps of the Bernese Oberland, the base is so permeated with the nearby villages that it is nearly invisibile to the people that visit it for the first time. If it were not for the futuristic control tower and for some helicopter parked in the large apron aside the main runway, it would be rather difficult to even notice its presence.

Unlike in Italy, the airport is surrounded by fences that are usually “symbolic”, sometimes almost absent, and from a system of level crossings activated during flight operations to prevent runway or taxiways incursions. Indeed, the local roads cross the field from North to South and from East to the West: a couple of lights regulate the crossing of the runway that is granted until a few moments before the take off of any aircraft. The base is a sort of large “open space”, with the locally based aircraft located inside the mountain shelters and maintenance hangars located in modern buildings along the main strip.

The airport is interested by heavy traffic of both jets and helicopters that operate in accordance with a standard “office timetable”: missions fly from Monday to Friday, both in the morning and in the afternoon, with a short break for lunch. Meiringen is the main operating base of the Fliegergeshwader 13 that is made up of two squadrons: the Fliegerstaffel (FlSt) 8, equipped of F-5, and the FlSt 11 that flies the F/A-18.

The base is wide and has free space to also accomodate transiting aicraft that are usually parked in the apron next to the threshold of the RWY 28 Obviously, being located in such a beatifull scenery, with two squadrons that fly many sorties each day, and free of the restrictions that characterize the majority of the European airports, Meiringen is a kind od “Eden” for spotters and photographers. The base has at least three main spotting points from where flying operations can be observed and photographed in favor of light.

To the Eastern one, it is possible to photograph the aircraft coming and going from the shelter area; more or less at the middle of the runway, it is possible to observe take-offs and landings, while the spotting point West is ideal for taxiing and departures from the nearby threshold of RWY 10. Even the Northern zone is full of spotting points, but they are less crowded because from those areas, aircraft are constantly backlighted. Indeed, sunlight enters the valley exclusively in “the warm” season, that is to say that from the end of October to April the Alps prevent the light from enlighting the base.

Usually, from September to mid spring, the tops of the mountains that surround on all sides Meiringen are whitened by the snow thus giving the alpine base a spectacular background.

Each spotting point has its own parking area: some are official and are clearly marked, others are improvised and require a simbolic fee (1 Swiss Franc) for the owner of the land. Dealing with the photografic equipment, the 100-400 mm range is optimal for the flying activity taking place in Meiringen, while a grandangular is ideal for taxiings.

The base is situated a few kilometers from the most famous range of the Swiss Air Force: Axalp Ebenfluh. The range, placed in a valley at 2.300 meters and made up of targets located on different slopes, is managed by a sort of small control tower that has the responsibility on the deconflictions of the formations involved in air-to-ground-shooting.

Once a year from 1942, in mid October, the Schweizer Luftwaffe opens the range to the public. The event is not limited to the shooting sessions of the Hornets and the F-5s but it comprises also Search And Rescue demonstrations, dogfight simulation (with huge use of flares) solo exhibitions and always ends with the display of the Patrouille Suisse.

The most important novelty of the 2006 edition of the Fliegerdemo was the participation of the Armée de l’Air, that took part in the shooting activity with a pair of Mirage F1 CR temporary deployed to Payerne. Even if this was not the first time a foreign air force used the Axalp range, it was the very first time foreign aicraft took part to the Demo.

For many years, the Fliegerdemonstration is carried out with the same timetable in both scheduled day (a wednesday and a thursday); should bad weather conditions occur causing a cancellation, the Demo is performed in a back-up date that is generally the following friday.

Obviously, in case of bad weather conditions, the event might be cancelled without prior warning. The display program is exactly the same in both days of the Demo, beginning at 2.00 PM local time and lasting more or less 1 and one half hours.

Actually, an interesting preview of the afternoon show takes place in the morning, when formations of F-5 and F-18 engage the range for the practice shooting sessions. Although such practices do not comprise the exhibitions of the Swiss team, neither helicopters or parachutists displays, the rehearsal session is even more spectacular than the Demo itself: for example, on October 12th, at the end of their morning training, the two French Mirage executed two “low and fast” passages on the crowd that will be long remembered.

Another reason to not lose the rehearsal is that before returning to base the aircraft generally execute a formation fly-by to the North of the Command Post allowing spectators and journalists to photograph the jets in favor of light as they fly along the Northern slope of the Axalphorn with the Brienz Lake in the background. The only problem is that to attend the morning practise, you must face before dawn (possibly without even knowing if the show will take place) a difficult and steep hike in the chill, that is even worse if undertaken with a bag full of photographic equipment, food and beverages.

In fact, unfortunately, the helicopters that enable VIPs and journalists to go up to Axalp from Meiringen begin their shuttle flights just after the end of the practice; consequently, the only way to reach the peak in time for the morning rehearsal is hiking.

Obviously, since you have to climb for a few kilometers, sometimes even in the snow, if you go up to the Axalp range on foot, you have to be very fit and dressed with a waterproof jacket, trekking boots and gloves.

As happened in the previous edition, 2006 display began with an AS 532 UL Cougar simulating an evasive maneuver with release of its entire load of 128 flares. Then the show went on with the dogfight of two Hornets (that finished their display with a steep climb to an altitude that rendered the condensation trails visible), with a SAR demonstration performed by a Alouette, with the shooting activities of F-18s, F-5s and Mirage F1s, a firefighting demostration, the solo display of the F-18 (piloted by Capt. Reiner the same who won the “Paul Bowen Single Jet Aerobatic Trophy” prize for the best solo display at RIAT), the interception of a “renegade” Learjet by two Horners and the exhibition of the Patrouille Suisse.

Despite the pain of the difficult hike, blessed with very good weather, Axalp Demo 2006 was once again a show worth seeing. For Axalp 2006 and Meiringen pics click here.

Condensation clouds & bang supersonici

Spesso chi guarda le foto che ho scattato ad Axalp mi chiede se le nuvole di condensazione che circondano i velivoli in manovra siano la manifestazione del “bang sonico” o di qualche altro fenomeno legato al volo in regime transonico. In realtà ciò che appare nelle foto fatte a 2.300 metri di altezza non è altro che l’effetto della rapida depressione sulle superfici di volo che porta il vapor acqueo dell’aria alla temperatura di condensazione. Si tratta di un fenomeno piuttosto frequente nelle manovre ad alto numero di G, durante le quali la depressione sul dorso alare aumenta notevolmente (come pure la portanza) , e che può essere osservato anche a livello del mare, a temperature dell’aria piuttosto calde, in condizioni di particolare umidità (come a Pratica di Mare il 17 settembre 2006) .


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Tutt’altra cosa sono le nubi di forma conica che si generano attorno ai velivoli in volo a velocità prossime a quella del suono. Attenzione, non si tratta di manifestazioni visive del cosiddetto bang sonico o del “superamento della barriera del suono”. Più semplicemente, si verifica che anche in volo livellato in regime transonico (quindi per velocità intorno a Mach 1.0) una qualsiasi convessità del velivolo (il canopy, le prese d’aria, ecc.) causa una rapida diminuzione della temperatura e della pressione e la conseguente creazione della nube.  La variazione di temperatura dovuta alla perturbazione del flusso prende il nome di Singolarità di Prandtl-Glauert. La particolare forma della nube associata alla singolarità è dovuta al fatto che in corrispondenza della perturbazione, il flusso può raggiungere velocità supersoniche ed essere associato quindi ad un’onda d’urto che si verifica allorché i filetti fluidi decelerano e la temperatura si alza rapidamente.  Ed è proprio lo shock, dovuto ad un brusco “salto” da una zona caratterizzata da bassa pressione, bassa temperatura e flusso supersonico ad una ad alta pressione, alta temperatura e velocità subsonica che viene percepito dal nostro cervello come un “bang”. In realtà, onde sfatare qualsiasi falso mito, è il caso di ricordare che il “bang supersonico” non ha niente a che vedere con il superamento della velocità del suono: si percepisce ogni qual volta un velivolo GIA’ in volo supersonico passa abbastanza vicino alle nostre teste da essere ascoltato. Il suono arriva improvviso a causa della velocità dell’aereo (che lo precede).
Un esempio di nube dovuta alla singolarità di Prandtl-Glauert è disponibile sul sito dalla US Navy (U.S. Navy photo by Ensign John Gay):