901 Locust Street 

Kansas City, MO 64106 

Subject: Honda Aircraft Company Petition for Exemption to 14 CFR Part 23, 

§23.181(b) for the Honda Model HA-420 Dynamic Stability Compliance; FAA 

Project TC9438AT-A 

Reference: (1) Honda Letter GN-9438-14-009 dated September 2, 2014 

(2) FAA Small Airplane Directorate Letter dated September 24, 2014 

Dear Mr. Lawrence, 

Reference (1) submitted in accordance with 14 CFR Part 11 Honda Aircraft Company's 

petition for an exemption to the requirements of 14 CFR Part 23, § 23.181 (b) for the 

Honda Model HA-420 as related to dynamic stability compliance. Reference (2) 

provided comments and request for additional information. 

Honda has updated our position, addressed your comments, and discussed this update 

with Mr. Lowell Foster from your office. Attached is our updated position. 

If you have any questions or comments on this request, please feel free to contact Mr. 

Clay Harden at (336) 662-0246 ext. 1433 or myself at (336) 662-0246 ext. 1670. 

Sincerely, 

Randy Griffith 

Honda Aircraft Company 

Division Director, Certification 

Enclosure 

Cc: 

Don Young, FAA Atlanta ACO 

HONDA 
regulatory compliance for certification of the HA

420 airplane during takeoff prior to the 

initiation of landing gear retraction and during landing with the airplane configured for 

landing. An exemption using a pilot evaluation is requested to confirm that no unsafe 

condition exists with the airplane's handling characteristics with the yaw damper 

disengaged during takeoff and landing. The Dutch roll damping requirements of 14 CFR 

Part 23.181 (b) at Amendment 23-62 would be met for the majority of the envelope 

including takeoff and landing at field elevations below approximately 5,000 feet. 

BACKGROUND: 

The Part 23 requirement for Dutch roll oscillations to be damped to 1/10 amplitude in 7 

cycles was written, in part, as a result of few small airplanes having yaw damper systems. 

In contrast, nearly all current 14 CFR Part 23 jet airplanes are type certificated with yaw 

dampers since the ride quality must be tailored to maximize passenger comfort and safety. 

Part 25 airplanes are typically flown by pilots type rated for that specific airplane. Small, 

non-turbojet Part 23 airplanes generally do not require a type rating, and this is the reason 

that the Part 23 requirements are and should be more stringent for airplane handling 

characteristics. 

Larger aircraft typically employ a more complex "series" yaw damper that responds 

without causing rudder pedal motion. Series yaw damper engagement may be employed 

throughout all flight phases including landing and touchdown. The HA-420 airplane uses 

a conventional reversible rudder control system, manually cable-driven via the rudder 

pedals. The "parallel" yaw damper utilizes an electronic rudder servo, which generates 

torque, moving both the pilot's pedals and the rudder to augment stability. The yaw 

damper is typically engaged in flight to assist in suppressing the Dutch Roll mode. 

However, the yaw damper also tends to suppress pilot crosswind rudder inputs, and 

increase the rudder pedal forces required to generate those inputs. Therefore, the yaw 

damper is disengaged during takeoff and landing to minimize workload. 

The in-service history of the 4 flight test aircraft (F1, F2, F3, F4) as well as the first 

production aircraft (P11) has shown that the airplane handling qualities are not 

objectionable with the yaw damper off during takeoff and landing. The quantitative 

analysis showed that the airplane damping is slightly less than the requirement to damp to 

1/10 amplitude in 7 cycles at high field elevations. 

CONCLUSION: 

The Model HA-420 lateral directional dynamic stability is positively damped and 

acceptable during expected operations with the yaw damper off. Operations of this type 

are very brief, and are limited to a small fraction of the operational flight envelope. 
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• The yaw damper is automatically engaged after takeoff and will automatically 

disengage upon landing. The AFM procedure will direct the pilot to manually 

disengage the yaw damper prior to landing. 

• During the short duration that the yaw damper is not engaged during each flight, 

the pilot is fully in the airplane control loop. The un-augmented dynamic stability is 

positive and any unwanted residual oscillations are easily controllable by the pilot 

Granting this exemption provides a level of safety at least equal to that provided under 14 CFR Part 23.181 (b) for the following reasons: 

• The yaw damper is required by AFM procedure and will provide a highly damped 

Dutch roll response for all flight phases when engaged. This will decrease pilot 

workload and improve safety. 

• Testing shows that, in the unlikely event of a yaw damper failure, the autopilot roll 

channel provides highly damped Dutch roll response at all altitudes up to the 

maximum operating altitude. 

• In rare instances when neither autopilot nor yaw damper are available, the AFM will 

provide a maximum "retreat" altitude where natural Dutch roll damping meets the 

basic rule (damping to 1/10 amplitude in 13 cycles). Company tests have shown, 

and certification testing will verify that the aircraft can safely descend to this retreat 

altitude from maximum operating altitude, and also descend from the retreat 

altitude to landing. 

• Relative to the objectionable workload with a parallel yaw damper engaged, 

disengaging the yaw damper for crosswind or gusty takeoffs and landings 

enhances safety. 

• As part of flight testing using normal procedures, company test pilots and DER 

pilots evaluated numerous takeoff and landings with the yaw damper disengaged. 

Testing consistently showed the natural Dutch roll damping resulted in acceptable 

workload and did not adversely affect safety. Furthermore, in the course of 

certification testing, FAA pilots will have ample opportunity to evaluate and confirm 

workload acceptability across the spectrum of atmospheric conditions naturally 

found during flight testing. 

Granting this exemption is in the public interest, for the following reasons: 

• For the airframe to inherently provide the required damping would result a less fuel 

efficient design. The aircraft was designed to cruise efficiently at high speed and 

high altitude. When the necessary level of safety can be provided, maximum 

efficiency is in the public interest 

• A more complex and expensive series yaw damper system design is not typical for 

this class of aircraft Years of industry fleet experience with this style of yaw 

damper show that the necessary level of safety can be achieved. Minimizing 

expense and complexity are in the public interest 
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internationally. Honda will coordinate with the applicable foreign authorities for validation 

requirements related with this exemption.