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  • Writer's pictureMike Lintott-Danks

Bell Geospace and the BT-67 Basler

Douglas DC-3 Dakotas have been flying since before the second world war but there are a number of these venerable aircraft that have been completely rebuilt, modernised and zero houred by Basler Turbo, these now fly for a variety of companies and air forces around the world. One of the companies that utilise this unique aircraft is Bell Geospace.

In 2021 Southampton airport saw two of their three BT-67 Basler’s visit, with one receiving some maintenance work at Jet Works Ltd and the other being based at the airport to complete survey work in Cornwall. Information about this survey work can be found at:

Airspeed Media were able to speak to Bell Geospace Field Supervisor Andrew Searle, via Zoom, to get first-hand account about the recent survey work over Cornwall and background information about the services Bell Geospace provide.

Survey operations overview statement by Andrew Searle

Bell Geospace operate Three Basler BT-67 DC3 conversion aircraft (see ; C-FTGX, C-FTGI & C-GEAJ to conduct low level aerial surveys. These are managed, maintained and crewed (2 Pilots and 1 ground based mechanic) by Enterprise airlines (see based in Oshawa, Canada and deployed and operated globally. There is also a Bell Geospace field supervisor and an operator deployed with the plane to manage the survey aspects and operate the science equipment to passively acquire (Gravity gradiometry and magnetic field raw data). The data is then further processed back at office to provide accurate 3D maps and associated documentation to quantify the sub surface density contrasts. We can either work exclusively for a client or we can acquire data on a multi-client basis which we can then sell. Each contract is different. There is a ‘global footprint’ map of where we have operated in the past along with technical descriptions of the planes and science equipment and further information on our website, LinkedIn page and our YouTube channel.

Once we have been awarded a contract, we need to set up our operations for that project; we comply to the International Airborne Geophysics Safety Association (IAGSA) guidelines. There are many processes we need to carry out to get a plane and crew into a country (Import/immigration/customs/work permits/in country CAA operating and low-level flying permits/environmental impact assessments and community engagements/Notams/FBO and fuelling/crew transport and accommodation to name but a few. We also need to have to provide a very effective briefing and establish a good working relationship and mutual understating of operations with the ATC (as we are frequently below the radio horizon so can’t always communicate with them but we typically provide them a bespoke flight following application login so they have constant visibility of where we are at any given time during flights)

We chose the Basler BT-67 as it is a very stable platform, can fly for over 6 hours, it can operate in almost any environment or type of runways, it can fly low and slow; on survey we fly at approx. 120 knots (flying lower and slower increases quality of data) at 80 metres above ground for land surveys and 120 metres above water for offshore surveys. We fly a ‘drape’ (imagine a virtual blanket thrown over the survey area), this covers and smooths out all the undulating landscape and obstacles, and is raised higher over built up or environmentally sensitive areas e.g. 300m or 500m, so we always have an increased safety margin. We then fly pre-determined gridded survey lines over the target area. We can acquire the best data when there is least amount of turbulence typically early in the morning (we are limited to VFR rules on survey so can only fly in the day) and offshore is much calmer than onshore; once we exceed a predetermined level of mild turbulence (or fuel endurance) we break off and return to base.

We are typically only a crew of five on a project so it is easier to manage the risk, especially in the current climate, compare this to say for example a marine seismic survey vessel with a large crew; more people to manage and they acquire data at approx. only 6 knots through active seismic charges and a lot of towed in water devices, we are totally passive (all sensors are within or on the plane) and can acquire the data 20 times faster. As we have a fully qualified AME with us, we conduct regular planned aircraft maintenance schedules (No 1, 2 and 3 inspections) at 150 flight hours intervals in the field; these are typically either solo or with minimal support in a local hangar to where we are operating. We carry with us a stock of spare parts and tools so that we are (fairly) self-reliant. When we ferry the plane between projects, as our top speed is only approx. 200 knots and endurance of 6 or 7 hours, we ‘hop’ our way around the globe a day at a time.

After we have done our mobilisation and completed our science equipment ground and air calibrations and commence survey, we would normally fly with the 2 pilots and either the field supervisor or field operator in the cabin to operate the science equipment. The vast majority of our stores and tools are offloaded and kept in the hangar. The other operator remains on the ground during flight to manage the recording of the ground science equipment and support the AME for ground handling and aircraft departure/return.

We have operated in many countries around the world conducting both onshore and offshore surveys. We have operated in various challenging environments (Malaysia at +40C and Canada at -30C to name but two)

C-FTGX and C-FTGI are both painted white to help reflect the heat, C-GEAJ is painted red/white/blue as it has spent some seasons in Antarctica. (Red shows up better against blue sky from below and Blue shows up better against ice/snow from above)


Once a days flying is completed, does the data get analysed that day and do you get informed if you need to survey the area again?

The analysts will have a quick look and they can look at different aspects and see if there has been a bad day for weather, like strong winds, that the information has been effected. They also look at the raw data but the actual data analysis can take weeks or months depending on the level of processing and then they build up the information for the client. It’s a bit like a jigsaw where you don’t have the picture and you only get a certain amount of pieces a day and then they are all put together from the more and more the aircraft flies and then the picture starts to reveal itself.

How long does it take from being awarded the contract to the start of the flying operations?

The best way to describe it is, how long is a piece of string, because there are many different things we have to contend with. It depends where the contract is around the globe, if there is an aircraft in that part of the world or whether we would need to reposition an aircraft. Sometimes it comes down to the client, where they want the work completed on a specific date. We also discuss with the clients about grouping the surveys together in on geographical location to complete them without the need to reposition the aircraft, which could save on costs. In country permits are another factor and we will apply to the Civil Aviation Authority of the country to be able to fly low level, prove we are airworthy and we have the correct insurance and crews have the correct training.

Is the work challenging for the pilots?

There is no auto pilot and the aircraft is always flown by hand whether it’s a survey flight or a ferry flight. The Captain and the First Officer share the work with one flying completing the survey work and the other is using the radio, navigating and looking out for obstacles and other aircraft. The navigation aids shows the box and survey area and lets the crew know if they are a few metres off the survey line, there is a tolerance though of 20 – 30 metres. Pilots are very experienced and take turns in flying due to the intensity of the work and break it up in either time or survey lines. Another reason for not using an autopilot is if the aircraft was off course the auto pilot would try to correct the course as quickly as possible causing issues with the data surveyed, but with the skilled pilots the move back towards the correct course is smooth allowing that data to still be used.

Why was the aircraft based at Southampton Airport?

The equipment is extremely accurate and takes a long time for it to calibrate and to get refined to the level that is needed. So, the important thing is we need to have very secure and very stable reliable power. Because we had been at Southampton for some maintenance work, that’s the reason we remained there, even though we had the work that needed completing in Cornwall. We did recce Newquay as a base but it would have meant a delay in the survey work by having to get generators and power supplies put in and all the equipment set up, it was easier to stay at Southampton.

Is there any scope to expand the fleet?

We have 3 aircraft and have 3 FTG systems which is what is needed. We have tried different aircraft, Cessna C208 Caravan, DHC-6 Twin Otter and we started with a Zeppelin. The data from the zeppelin was fantastic but it was quite slow and took a long time to change directions. Also there is a lot more time for set up with regards to the Zeppelin. The Basler can carry all the equipment and spares we may need and travel for 6 -7 hours in hops across the world.

Where is the most interesting place you have flown?

That’s a great question. One of my first projects I was in Zambia flying over lakes and jungle and it was incredible. The first time I did a solo flight, as an operator, was over Zanzibar which is a beautiful location and again in Africa flying over Lake Tanganyika with scrubland, cliffs and mud huts along with the wide variety of wildlife. Another was a flight in Manitoba with temperatures as low as -40c but that was absolutely stunning, and the weather was fantastic. One of the other things I like are the ferry flights with one trip starting from Muscat in Oman to Kuwait, then to Shamel Sheik and onto Kalamata Greece. Next was Memmingen in Germany, Wick in Scotland then to Iceland and onto Greenland. The change in temperature from Oman which was +40c and then to Greenland where it was -35c. The final hops were to Happy Valley, Goose Bay Canada and finally the last stop to Oshawa, Canada. It is a challenge to ensure that we have all the correct clothing as there is always a chance that you could ferry somewhere that is a completely different weather than where you are at that time. The aircraft is not pressurised and when above 10k feet oxygen masks are worn and there are heaters but it’s not a commercial airline, so the cold weather clothing is essential. (transit van with wings).

Is there much down time of the aircraft?

There isn’t a great deal of downtime and there is a mechanic with us on each project. If there is it is not just the airframe that could be the cause as the equipment could be not working correctly but this is another reason for the Basler being our aircraft of choice as we can carry spares with us to repair many of the common issues. We try to be as self-sufficient as possible. Any downtime is used efficiently as the pilots need a rest day in every 7 days so this is utilised when an issue takes 2- 3 days to rectify and we complete other planning work whilst waiting for parts to arrive. An example is we were once in Zambia and we were operating out of an old WWI RAF gravel airstrip and it still had the original control tower and Nissen huts and the parts took two days to arrive and then half a day to complete the repair and during that time we completed some routine maintenance to the aircraft and science equipment.

Do you help support the mechanic and crew in the repairs/maintenance?

We do to a certain extent with some of the repairs/maintenance, for example in Southampton we needed to fit the de-icing boots and the mechanic, two pilots, myself, and an apprentice mechanic (from Canada) completed this work with myself and the pilots supporting the mechanics by shifting and lifting what they needed. We all help and support each other where needed creating greater team cohesion.

Thank you to Julianne Sharples for arranging the interview and especially Andrew Searle for discussing at length the specialised work completed by Bell Geospace. Also a big thank you to Stef and Wes for allowing access to the stunning C-GEAJ BT-67 after it arrived at Southampton Airport to continue the survey work over Cornwall in March 2022.

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