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At about midnight, I saw a small plane (a Diamond Twin Star) take off from Heathrow, on flightradar24. It then proceeded to fly in irregular loops around North London, at low altitude, 1700ft. The plane had registration G-COBS, and seems to be owned by "Cobham Flight Services", which provides "flight inspection services". Their web page advertises how good they are, without explaining what they do.

It seemed unusual, as such small planes don't normally use Heathrow, and flying low over a city at night also seems unusual. What was the purpose of a flight like this?

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  • $\begingroup$ I saw this exact same thing. Such weird flight patters and seems to have landed near RAF Northolt. I can only assume it was some sort of military testing. They are know to scan parts of London and zap electronics data from below. I guess "flight inspection" is the perfect cover. $\endgroup$ – MajorMajor Feb 8 '17 at 14:05
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    $\begingroup$ If you're interested in the subject, you might enjoy this article, where a reporter goes along for a ride on FAA flight check operations. It apparently involves a lot of maneuvering and no small amount of nausea, not to mention birdstrikes. $\endgroup$ – Zach Lipton Feb 18 '17 at 20:52
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    $\begingroup$ @MajorMajor -- there's really nothing sekret sqwirl about what's going on with navaid flight inspections -- think of it as a test flight to make sure the navaid works, just like your mechanic would test drive your car after doing a bunch of work on it to make sure that all the work is good and he didn't put it back together wrong :) It's just that the stakes are higher for navaids... $\endgroup$ – UnrecognizedFallingObject Feb 19 '17 at 12:53
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Flight inspections are flights performed to measure the performance of the communication, navigation and surveillance (CNS) infrastructure of air traffic control.

Flight tests are required to examine the signals-in-space as received at the aircraft after being influenced by external factors such as site conditions, ground conductivity, terrain irregularities, metallic structures, propagation effects, etc.

source: ICAO DOC8071

In this case the inspection flight seemed to be performed to inspect the performance of the Instrument Landing System at RAF Northolt. The aircraft did numerous approaches on runway 25 and flew across the localizer twice at approximately 11 km (6NM) from the runway threshold. This is a normal procedure for ILS testing.

enter image description here

For those interested in the details of the ILS testing procedure seen on FR24, it is described in section 4-10 of ICAO Document 8071, Manual on Testing of Radio Navigation Aids, Volume I (Testing of ground-based navigation systems).

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From their website,

Cobham conducts commissioning, return-to-service, engineering and periodic flight inspections in addition to trials work on a range of systems including, CAT I/II/III ILS, MLS, PAR, DVOR, NDB, U/VDF, TACAN, DME, AGL and PAPI and PSR/SSR.

The specialist capabilities include procedure validation, ADS-B and MLAT-systems, ship borne navaids, FI programme management, technical investigations, communication tests and trials, windfarm trials, flight inspection advice, and training.

Cobham has formal approvals from UK CAA Safety Regulation Group (CAP 670), UK MoD, INAC (Portugal), ENAV (Italy), British Standards Institute (BS EN ISO 9001:2008), Germany BAF, and Air Safety Support International OTAR for UK Overseas Territories.

It looks like they test airport/en-route equipment such as approach systems and ADS-B systems etc. This can presumably be done in any aircraft equipped to fly/test those approaches and systems.

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  • $\begingroup$ The following link provides some information and history of Flight Inspection from the FAA. $\endgroup$ – Gerry Feb 8 '17 at 13:32
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I live nearby Frankfurt airport and every few months or so the DFS (German ATC) issues a press release about inspections flights.

Sample from their homepage:

Flugvermessung am Flughafen Frankfurt (Flight inspection at Frankfurt Airport)

Noise disturbance to be expected during the day and at night

14.11.2016.- In the period from 17 to 20 November 2016, calibration flights will be conducted at Frankfurt Airport.

The accuracy of the instrument landing systems of the southern runway and the centre runway will be inspected for landing direction 25 (westerly weather patterns) of the southern runway and 07 (easterly weather patterns) of the centre runway. The calibration aircraft being used is a twin-turboprop Beechcraft Super King Air 350. During this time, noise disturbance is to be expected during daytime and nighttime hours.

Despite the night curfew at Frankfurt Airport, technical flight checks such as calibration flights may also be conducted at night as they would have a great impact on regular air traffic during the day.

The calibration of technical facilities is indispensable for the safety of air traffic. DFS would like to apologise for any inconvenience caused by such flights.

This one was for RWYs 25L and 07C but three months earlier they did similar flights for RWY northwest (07L/25R). It's interesting (and attracting) that flightradar24 shows the flight around LHR from your question. Next time I hear about inspection flights in my area I'll have a look at flightradar24 too.

Update: A few days ago DFS announced further calibration flights from 16 to 19 February 2017 around FRA/EDDF to inspect ILS accuracy for RWY northwest (07L/25R). I figured out they use (at least) two aircraft, D-CFMB and D-CFMD. This time it was the latter. They flew on Feb 16 (left) and Feb 17 (right):

Flight inspection Feb 16 at FRA Flight inspection Feb 17 at FRA

But the funniest one must have been on Feb 15 from DRS (Dresden) to Berlin (SXF), rendering kinda crop circles:

enter image description here

All pictures from FR24.

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From the Flight Inspection Wikipedia entry:

Flight inspection refers to the periodic evaluation of navigational aids used in aviation, such as flight procedures and electronic signals, to ensure they are safe and accurate. Unlike flight tests, which analyze the aerodynamic design and safety of the aircraft itself, flight inspection comprises reviewing flight procedures (such as routes, approaches and departures) to ensure navigational support is sufficient, there are no obstacles and the procedure is reliable.

It may seem unusual, but it is actually a very common procedure, done regularly as part of national and/or international aviation standards, and every time there is a significant change to ground infrastructure which may affect Nav Aids signals.

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