AIRCRAFT MODIFICATION– FLIGHT TEST CONSIDERATIONS

INTRODUCTION

Contents

  1. Introduction
  2. Types of Modification
  3. ModificationApproval Process
  4. Flight Test Requirements
  5. Flight Test Methods
  6. Flight Test Safety

Annex A – Terms and Abbreviations

Annex B – Certification Flight Test References

References

  • Part 21 of CASR 1998, Certification and Airworthiness Requirements for Aircraft and Parts.
  • CASA Advisory Circular 21-13(0) – Australian Designed Aircraft – Type Certification, September 1999.
  • CASA Advisory Circular 21-15(1) – Supplemental Type Certificates, September 2009.
  • CASA Advisory Circular 21-40(0) – Measurement of Airspeed in Light Aircraft – Certification Requirements, April 2005.
  • CASA Advisory Circular 23-1(0) – Airspeed Airworthiness Standards for the Installation of Equipment that Protrudes into the Airflow, 2013.
  • CASA Advisory Circular 21-47(0) – Flight Test Safety, April 2012.

Purpose

1.This paperhas been produced in order to provide somebackground information and guidance regarding the flight test considerations relating to modifying aircraft in accordance with Civil Aviation Safety Regulations (CASR) 1998 Part 21 Subparts E and M. People responsible for the certification and flight testing of aircraft incorporating such modificationsmay find it useful.

Acronyms

2.Terms and abbreviations are listed at Annex A.

Definitions

3.For the purposes of this document:

Certification Flight Testing means those flight tests conducted for the purpose of demonstrating, or verifying, compliance with the applicable airworthiness standard.

Experimental/Developmental Flight Testing means those flight tests conducted for the purpose of defining or expanding an aircraft’s flight envelope.

Flight Test means the process of developing and gathering data during operation and flight of an aircraft and then analysing that data to evaluate the flight characteristics of the aircraft (Sub regulation 21.35 of CASR 1998 refers). In this paperflight test does not mean a practical test of a person’s aeronautical knowledge and flying skill (as per Part 1 of CAR 1988).

Background

4.Australian aircraft are often subject to modification under CASR Part 21. This can occur to examples that have either been type certificated in Australia or certificated overseas and then accepted for use in this country through the issue of a Type Acceptance Certificate (TAC). Others, for instance, amateur-built aircraft operating under the Experimental Certificate system, may also be exposed to modification projects. The Supplemental Type Certificate (STC) process is one method under which design approval can be granted – Advisory Circular (AC) 21-15(1) refers. Fully type-certificated aircraft, once modified,must continue to meet the requirements of their certification basis and the associated airworthiness standards. With those modifications that have flight type implicationscontinued satisfaction of the relevant clauses of the airworthiness standards will probably require some form of flight testing or assessment be conducted. The aims of this paper are to provide background information on modification flight testing requirements and to offer guidance regarding the procedures and techniques that can be employed. While the requirements are applicable to all aircraft the information provided is focussed toward flight testing of those at the lighter-weight and lower-speed ends of the scale, principally those in the primary, normal, utility or acrobatic categories.

TYPES OF MODIFICATION

5.Those modifications that may have flight type implications would include any that influence the performance or handling characteristics of the aircraft, any systems changes liable to affect aircraft operation or have consequences for the pilot and crew, and anything that requires amendment to the Aircraft Flight Manual (AFM).

6.Performance and Handling Characteristics. Modifications that are likely to affect an aircraft’s performance or handling characteristics are as follows:

a.Any piece of equipment that protrudes into the airflow or any alteration to the basic shape of the airframe. These types of modification normally introduce an increase in the aircraft’s overall drag index and will consequently degrade performance capabilities. Stability and control characteristics may also be alteredif the wake of the attached equipment affects the airflow over the wing or empennage, and especially if it impacts downstream flight control surfaces. The creation of such effects is not necessarily restricted to large protuberances; even small attachments, if strategically located, may generate significant vortices. Another important consideration relates to any changes to the airflow over, or the pressure field around, pitot tubes or, more significantly, static ports. Such effects would require a recalibration of the aircraft’s airspeed-indicating and static pressure instrumentation (see paragraph 12.a below). Examples of protuberance type modifications would be:

(1)Camera, laser or radar pods (or other similar stores) mounted on aircraft wings, struts, skids or undercarriage legs, etc.

(2)Cargo pods or panniers, including those typically fitted to the underside of an aircraft’s fuselage.

(3)External fuel tanks.

(4)Agricultural or fire-fighting spray equipment.

(5)Radio or avionics antennae.

(6)Nose booms, tail-stingers, or equipment mounted at wing-tips.

(7)Ski or float landing gear.

b.Wing or profile modifications intended to improve the aerodynamic qualities of the aircraft. Such modifications would include:

(1)Changes to the size, profile or shape of wings, wing-tips, empennage or rotors.

(2)Introduction of fences, vortex generation (VGs) or boundary layer control equipment.

(3)Drag reduction – for example, fuselage fairings, undercarriage spats.

c.Any changes to flight control systems. For example:

(1)Changes to primary flight control surface, or tab, size or profile.

(2)Changes to flight control surface range or rate of movement.

(3)Tab-gearing ratios, spring sizes, control runs, etc.

(4)Changes to automatic flight control, auto-pilot and stability augmentation systems.

d.Engines and propellers. Any modifications or changes to the powerplant or propeller installations that involve a change in power output will affect the performance capabilities of the aircraft and can also easily affect handling and stability characteristics. Propeller changes can include alterations to slipstream and gyroscopic effects.

e.Weight and/or CG. Proposals to change, normally to increase, an aircraft’s maximum take-off weight (MTOW) will affect performance and possibly affect some handling characteristics. Proposals to alter, increase, centre of gravity (CG) ranges or limits will probably alter handling characteristics and may affect some performance figures. Changes to other weight limits, e.g. maximum landing weight (MLW), may also require flight type consideration.

7.Systems Changes. Popular modifications are those that involve the replacement, improvement or addition of aircraft systems. Any systems changes that have implications in the cockpit or that otherwise affect the way the aircraft is operated will require flight type assessment to ensure continued compliance with the requirements of Subparts D, E, F and G of the applicable airworthiness standard. Assessment in these cases normally involves human factors (HF), operability and effectiveness evaluations. Some systems modifications can also result in flight characteristics changes. Both quantitative and qualitative assessments may be required. Specific consideration will be included with modifications that affect operations under the Instrument Flight Rules (IFR) with complications often raised should flight into known icing conditions be involved. Systems that can be subject to modification include the following;

a.Cockpit controls.

b.Engines, propellers, fuel systems.

c.Hydraulics, landing gear, steering, brakes.

d.Flight instruments.

e.Electrics.

f.Avionics and communications.

g.Integration and automatic flight systems.

h.Electronic flight bags.

i.Cockpit environment, pressurisation, ventilation.

j.Lighting.

k.Fields of view.

l.Seating, harnesses.

m.Cabin environment, entry and exit.

8.Aircraft Flight Manual. Many modifications require an amendment to the Aircraft Flight Manual (AFM) or the generation of a Flight Manual Supplement (FMS). Flight test or operational aircrew should be involved with the writing and review of these documents.

MODIFICATION APPROVAL PROCESS

9.The certification approval processes for aircraft modifications follow those for the aircraft itself – AC 21-13(0) refers. Continued compliance with all requirements of the original, or updated, certification basis[1], including all clauses of the applicable airworthiness standards, must be redemonstrated, or at least reclaimed. CASA, or an Authorised Person, must then agree that compliance has been found. AC 21-15(1) describes the certification process relevant to major changes to a type design approved under an STC. It also makes reference to changes that can be approved under CASR Part 21M. In either case the same basic certification process applies, the essential difference relevant to approvals under Part 21M is the reduced involvement of CASA itself.

FLIGHT TEST REQUIREMENTS

10.Modifications will require individual analysis and the flight testing requirements relating to the certification of each modified aircraft may range from nil to substantial. In all cases the flight test phases are the same as those applicable to accreditationof the aircraft itself. Firstly,the applicant is responsible for developmental flight testing and then for the testing required to demonstrate or claim continued compliance with the relevant clauses of the certification basis. CASA, or an Authorised Person, may then conduct additional flight tests to validate the applicant’s results and compliance claims.

11.Compliance Matrix. A compliance matrix that includes all relevant clauses of the applicable certification basisshould be generated. Often, in an attempt to save on paperwork, a compliance matrix for a modification project will be limited to only those clauses adjudged relevant. However, unless the modification is one that has absolutely no flight type implications(for example, an internal structural change that does not affect the aircraft’s weight and balance limits)this matrix is better off including all flight type clauses. Occasionally, a change in the parameters satisfying one requirement can have a ripple effect. Those clauses that are then, indeed, found to be unchanged can be designated Not Applicable (N/A). Or, in cases where compliance is being claimed through some method other than flight test (e.g. analysis or similarity) the reasoning can be included, or referenced, in the comments column of the matrix. Thus, a complete analysis of the certification basis is presented to the certification authority and the possibility of overlooking an applicable requirement is minimised.

12.Noteworthy Items. Sometypes of modification with flight-type implications can draw special attention when testing requirements are considered. As follows:

a.Pressure Error Corrections. Any modification that changes the pressure field around the aircraft has potential to affect the errors introduced to the aircraft’s airspeed and altitude measurement systems. This is particularly relevant for protuberances or attachments upstream of, or near, the aircraft’s static ports. Should there be any question that the airflow around the static ports will be altered by the modification then an airspeed system recalibration should be carried out (calibration requirements are discussed at AC 21-40(0) – Measurement of Airspeed in Light Aircraft – Certification Requirements). Major changes to the PEC data may then require redefinition of the operational airspeeds presented in the AFM or FMS. Changes to V-speeds will also need to be taken into account during any performance and handling testing that is required as a result of the modification.

b.Performance Requirements. Modifications which are likely to cause changes to the performance capabilities of an aircraft can be considered from three aspects:

(1)Where a modification is expected to cause a minor but noticeable increase in drag, which is then likely to result in a small decrement in performance, the certification authority may accept areduced testing philosophy to verify the predicted outcome. This would entail the accurate measurement of the decrement as it applies to the most criticalperformance requirement followed by the conservative factoring of related requirements based on the first set of test results. Such would depend on the basic, unmodified, aircraft comfortably meeting all the relevant certification class limits; for example, the maximum stall speed or minimum climb rate requirements stipulated in FAR 23 for a normal category aircraft.

(2)Where a modification may result in a small but noticeable increase in performancethe certification authority may again accept a claim of compliance with the performance clauses based on the reduced testing philosophy, provided the applicant does not plan on claiming any credit. In this case, the original figures would be retained in the AFM even though the modified aircraft would be capable of outperforming them.

(3)Significant changes, or any claims for improvement, in performancewould require a comprehensive retesting of the modified aircraft such that continued compliance with the standards can be confirmed and accurate data can be provided to the operational pilot via an AFM amendment or FMS.[2]

c.Spinning. Single-engine normal, utility and acrobatic category aircraft must meet the various spinning requirements set down in clause 221 of the applicable airworthiness standard. Spin flight testing can be an arduous exercise involving elements of elevated risk. For modifications with limited effect on the flying qualities of the aircraft the certification authority may accept a claim of compliance based on analysis as long as the other handling qualities requirements, especially the stalling characteristics, of the modified aircraft have been tested and been shown to have suffered minimal effect. Nevertheless, spin testing of the modified aircraft is recommended in the following circumstances:

(1)Moments of inertia have changed by more than approximately 5 per cent.

(2)Power available increases by more than approximately 10 per cent.

(3)The modification introduces major changes to the aircraft’s flight control system.

(4)The modification includes the incorporation of canards.

(5)The modification introduces major wing changes.

(6)The modification introduces major changes to the airflow over the aircraft’s empennage.

d.Vibration, Buffet and Flutter. Flight testing for any modification that is likely to affect the flight characteristics of the aircraft should include checks that compliance with the vibration and buffet requirements has not been compromised. This is especially relevant where the modification includes an attachment or a protuberance into the airflow. In these cases testing should assess both the effect the modification may have on the vibration characteristics of the aircraft overall, as well as the vibratory behaviour of the modification itself. Specialised cameras and other flight test instrumentation may be required if direct observation of the mod is not possible. Flutter characteristics should be reassessed after major modifications particularly if the aircraft structure, the airflow over control surfaces, or the flight control system itself, has been altered.

13.Limit Airspeeds and Flight Envelopes. When flight testing a modified aircraft the general aim would normally be to check that it continues to meet certification requirements out to the edges of the flight envelopes (e.g. weight / CG; airspeed / altitude) originally applicable to the basic aircraft (sometimes known as Limit Basic Airframe (LBA)). Nevertheless, some modifications may require adjustment to the LBA envelopes as follows:

a.Contraction. Modificationsmay affect the flying qualities of the aircraft such that, for compliance with the requirements of the airworthiness standard to continue to be met, additional limitations, more stringent than those established for the basic aircraft, must be imposed. For example, certain handling requirements may only be acceptableif the aft limit of the CG range is moved forward, or performance requirements satisfied if weight constraints areapplied, or vibration and buffet requirements met if maximum speed restrictions are introduced. All limitations associated with a specific modification, or combination of modifications, must be provided in the FMS. In the case of speed limitations the inclusion of a cockpit placard is recommended – e.g.Maximum Airspeed when fitted with the ACME Camera Pod – 100 KIAS. See also AC 23-1(0).

b.Expansion. Modifications, significantly engine changes, which allow for substantial increases in performance (as per paragraph 12.b(3) above) may well also require an expanded redefinition of operational limits after results of the flight test program are consolidated. In such cases the FMS must contain all new limits and cockpit placards should be changed. Applicants should also be aware that should a substantial increase in capability be achieved, such that the design cruising speed (VC) is increased, then a revision of the velocity-load factor diagram may be required (cf, for example, FAR 23.333 and 23.335). In these cases a recertification exercise resulting in a change to the TC is probablywarranted.

FLIGHT TEST METHODS

14.Any certification flight test program that is to be conducted on a modified aircraft must be completed using the guidance material relevant to the certification basis, or using agreed flight test techniques and data reduction methods of equal engineering rigour. A list of some relevant references is provided at Annex B.

15.While the same methods and test techniques are applicable there may be scope for simplification in the way in which they are applied. The use of comparative tests is one useful method which can be especially applicable to the modification of a single aircraft under CASR Part 21M. In these cases, the particular aircraft, which is often already well into the ageing aircraft class, is tested in its unmodified state and the detailed results recorded. The modification is then incorporated and the aircraft subjected to the same flight tests again under, as nearly as possible, the same test conditions. The two sets of results are compared and the effects of the modification defined. Additionally, depending on the projected magnitude of the effects of the modification the certificating authority may accepttesting at the most operationally demanding conditions followed by some extrapolation to other points about an envelope.

16.This comparative method should only be applied in cases where compliances with class limits are not being threatened. If doubts about such certification basis requirements exist then the flight test examination of the modified aircraft should be conducted against the specific airworthiness standards clause and not just the capabilities of the unmodified aircraft.

FLIGHT TEST SAFETY

17.The information provided at AC 21-47(0) – Flight Test Safety is applicable.

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ANNEX A

TERMS AND ABBREVIATIONS

Symbol/Term / Definition
AC / Advisory Circular
AFM / Aircraft Flight Manual
CAR / Civil Aviation Regulations (1988)
CASA / Civil Aviation Safety Authority
CASR / Civil Aviation Safety Regulations (1998)
CG / Centre of Gravity
CoA / Certificate of Airworthiness
FMS / Flight Manual Supplement
FTS / Flight Test Schedule
HF / Human Factors
ISA / International Standard Atmosphere
LBA / Limit Basic Airframe (or Aircraft)
MLW / Maximum Landing Weight
MTOW / Maximum Take-Off Weight
PEC / Pressure Error Correction
TAC / Type Acceptance Certificate
TC / Type Certificate
TP / Test Pilot
VG / Vortex Generator

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