Issue 02 – Negative torque sensing system (NTS) - Civil Aviation Safety Alerts (CASA) No. 2014-04


To all owners, operators and maintainers of Honeywell TPE331 series engines

Issuing Office: National Aircraft Certification
Document No. : CASA 2014-04
File Classification No. : Z 5000 35
Issue No. : 02
RDIMS No. : 99456688
Effective Date: 2014-10-22


To emphasize to the owners, operators and maintainers of Honeywell TPE331 series of engines that:

The NTS System fitted on the Honeywell TPE331 engines is not an automatic feathering system. It will function only when certain preset conditions and parameters are met, it is therefore, not always available;

Under certain conditions and settings, such as retarding the Power Levers from the forward position, misrigging of the NTS System or improper fuel valve setting, the NTS System may malfunction or not function at all;

The primary reason of the NTS System is to provide propeller drag reduction by driving the propeller blade angle towards feather at the optimal rate in case of an engine failure or shut down, and the secondary reason is to assist in the air start;

The NTS System may induce a false sense of feathering. Flight crew must guard against this perception and be aware that full feathering can only be achieved by manually feathering the propeller;

Asymmetric drag, under certain condition and configuration could cause the loss of control at speeds above Vmca, especially in the landing configuration when the propeller of the failed engine has not been feathered.


The NTS system description in the aircraft flight manual (AFM) and pilot operating handbook (POH) may cause the flight crew to incorrectly believe that it is an automatic feathering system; is always available; and will always activate in the event of a power loss or engine shutdown.  This misconception has not been clarified during the pilots training sessions.

The NTS drag protection system is designed to reduce the urgency with which the pilot is required to feather the propeller of the failed engine and allows him/her to concentrate on controlling the aircraft. Misconception of the NTS System has distracted the flight crew from carrying out the appropriate emergency procedure. Flight crew must always be aware that where the propeller is required to be feathered, it can be done only manually and they cannot expect the NTS System to do it.

The operation of the NTS System has misled the flight crew into believing that the propeller has been feathered. This has distracted them from manually feathering the propeller, contrary to the prescribed emergency procedures. Consequently, when Power Levers were retarded it interrupted the drag protection provided by the NTS System and surprised the crew with asymmetric drag. If this happens during the approach, the unexpected asymmetric drag could startle the pilots and result in loss of control.

Recommended action:

The owners, operators and maintainers of Honeywell TPE331 series of engines must ensure that:

  1. The AFM, POH, standard operating procedures (SOP) and Supplementary Materials, as applicable, must clearly establish that the NTS System is not designed to automatically feather the propeller but only to provide drag protection; where the propeller is required to be feathered it can only be achieved by manually feathering it.
  2. The AFM, POH and SOP must be reviewed in conjunction with Honeywell published Pilots Tips for Honeywell TPE331 Revision 1 dated October 2009; Operating Information Letter (OIL) OI331-20R1 dated March 10, 2009; or Operating Information Letter OI331-21R1 dated March 10, 2009 (as applicable). Though AFM and POH over-ride the Pilot Tips and OIL, it must be ensured that these documents are aligned and any apparent disparity must be clarified by the engine manufacturer. Subsequently any affected SOPs must be amended to comply with the manufacturer's instructions.
  3. The initial and annual technical ground and flight training program for flight crew operating these engines should contain a detailed syllabus on the NTS System, including its normal, abnormal and emergency operations. Any misconception concerning the operation of the NTS system must be clarified during these training sessions.

Secondary recommendation

It must be emphasized that in any engine failure situation, flying the aeroplane is the paramount important action; followed by the memory items and then the checklist items.

The pilot must avoid windmilling within the Shaft Critical RPM Range as this may cause damage to the engine.

Avoid windmilling in the wrong direction (propeller rotation reversed) due to excessive sideslipping as this may cause damage to the carbon brushes in the Starter/Generator. Slideslipping may be induced by excessive rudder deflection or trim in the wrong direction.

Effective date

Effective on the date of publication

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact:

Sudhir Kidyur
Civil Aviation Inspector
Commercial Flight Standards – AARTFS
NCR, Ottawa, ON K1A 0N5
Phone:  613-990-1495
Fax:  613-990-6215


Aaron McCrorie

Director | Directeur

Standards Branch | Direction des Normes

The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.