The de Havilland Chipmunk T.Mk.20 variant was specifically built for export military customers, featuring modifications suited for training roles such as enhanced instrumentation and military-specific equipment. However, as these aircraft aged out of military service, many were converted to civilian standards for continued use in flight training, aerobatics, and recreational flying. The civilian Mk.22 standard represents a converted ex-military de Havilland Chipmunk, adapted for civil certification while retaining much of the original airframe’s integrity. This conversion process not only extends the life of these historic aircraft but also ensures they comply with modern civil aviation regulations.

Converting a T.Mk.20 to Mk.22 involves a meticulous series of inspections, modifications, and certifications, guided by established design standards like British Aerospace Drawing No. C1-G73 Issue 2.

Explores the step-by-step process, highlighting the technical details, regulatory requirements, and practical considerations for aviation enthusiasts, restorers, and operators.

Understanding the Variants: T.Mk.20 vs. Mk.22

To appreciate the conversion, it’s essential to distinguish between the variants. The T.Mk.20, produced at de Havilland’s Hatfield facility in the UK, was an export version of the RAF’s T.Mk.10. It was equipped with a 145-hp de Havilland Gipsy Major 8 engine (military designation), a two-bladed fixed-pitch propeller, and 9 Imperial gallon fuel tanks per wing. Military features included VHF radio sets, gunsight mounts (though unarmed), IMC-flying instrumentation, and sometimes anti-spin strakes for improved handling during training maneuvers.

In contrast, the Mk.22 is the civilian conversion of ex-military models like the T.Mk.10 or T.Mk.20. The de Havilland Chipmunk Mk.22 retains the original 9-gallon fuel tanks but undergoes modifications to meet civil airworthiness standards. A related variant, the Mk.22A, upgrades to 12-gallon tanks for extended range. The primary goal of the Mk.22 conversion is to remove military-specific components, restamp the engine to a civil Gipsy Major 10-2 designation, and incorporate civil avionics and safety features.

Key differences include:

• Engine Designation: Military Gipsy Major 8 becomes civil Gipsy Major 10-2 via restamping and potential overhauls.
• Fuel Capacity: Retained at 9 gallons for Mk.22, optional upgrade for Mk.22A.
• Equipment: Removal of military radios, IFF transponders, and addition of civil navigation aids.
• Canopy and Aerodynamics: Optional blown canopy for better visibility and wing luggage compartments for utility.

These changes transform the aircraft from a dedicated trainer to a versatile civil aircraft capable of aerobatics, touring, and other arial jobs including glider towing.

Regulatory Requirements and Preparation

Before commencing conversion, compliance with aviation authorities is key. In the UK, the Civil Aviation Authority (CAA) oversees the process, requiring adherence to British Civil Airworthiness Requirements (BCAR) or EASA standards for European operations. The approved design standard for converting the de Havilland Chipmunk T.Mk.20 aircraft is outlined in British Aerospace Drawing No. C1-G73, which details the necessary modifications for civil certification.

In the US, the FAA may issue a Supplemental Type Certificate (STC) for imported Chipmunks, while in Australia, the Civil Aviation Safety Authority (CASA) accepts certain unconverted military models but prefers full civil conversions.

Step-by-Step Conversion Process

The conversion, typically performed by certified maintenance organizations or specialized restorers, follows a structured approach. While exact details are proprietary to Drawing C1-G73, common steps based on historical conversions include:

1. Disassembly and Removal of Military Components

• Strip the aircraft of military equipment: Remove VHF radios, gunsight brackets, and any armament wiring.
• Inspect and replace wiring harnesses to civil standards, eliminating redundant military circuits.
• This phase ensures the aircraft meets civil weight and balance requirements, often reducing empty weight by 50–100 pounds.

2. Engine and Propeller Modifications

• Restamp the Gipsy Major 8 to Gipsy Major 10-2, involving paperwork and potential cylinder head changes.
• Upgrade the starter from Coffman cartridge to electric (Mod H.378), improving reliability.
• In some cases, changing to a constant-speed propeller for better performance, though the original fixed-pitch is often retained for cost savings.

3. Airframe and Systems Upgrades

• Fuel System: Retain 9-gallon tanks or upgrade to 12-gallon for Mk.22A variant.
• Canopy: Fit a blown Perspex canopy for enhanced visibility and aerobatic suitability.
• Undercarriage: Reinforce legs and add fairings for drag reduction.
• Avionics: Install modern civil radios, GPS, transponder, and ELT. Instrument panels are updated to civil specs.
• Aerodynamic Mods: Add anti-spin strakes (Mod H.231) and optional wingtip tanks or luggage bays.

4. Painting, Testing, and Certification

• Repaint in civil schemes, removing military markings, unless authorized to maintain military livery.
• Conduct ground runs, weight-and-balance calculations, and test flights to verify handling.
• Final inspection by authorities leads to a civil Certificate of Airworthiness.

The entire process can take 6–12+ months.

Irish Historic Flight Foundation Chipmunks

The IHFF currently operates 3 de Havilland Canada DHC-1 Chipmunks, “168” (EI-HFA), “169” (EI-HFB) & “170” (EI-HFC). These aircraft are often seen in tight formations operating at national and international events.

Two of our three DHC-1 Chipmunks originated from the RAF, with a the remaining Chipmunk being a former Irish Air Corps aircraft, all of which have gone through military to civilian conversions. The IHFF Chipmunks, whilst civil aircraft, operate in historic Irish Air Corps livery and markings.