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BuiltByNOF

 

Australian Comanche Society

Pilot Proficiency Programme

Critical Maintenance Items for Pilots

By Roy Sneesby and Ken Holdsworth

 

As well as all of the safety-of-flight items typical to the smaller GA aircraft, there are a number of critical maintenance and system related items that the prudent Comanche pilot should keep in mind. Most of these should be checked on each preflight inspection and all require attention from your engineer during the 100 hour or annual inspections.  This list is not exhaustive and we assume that your Comanche is being maintained in a proper manner by experienced Comanche technicians. All of the items in the following tables have caused incidents many of them fatal.

 

Legend: It Item. Pr problem. Im - Impact. App applicability.

 

It: Tail

Pr.: Loss of Stabilator Trim tab actuator arm bolt and nut

Im.: If this bolt falls out, the actuator arm will separate from the trim tab, which will float freely. Since the Comanche has an "all flying tail" the result is total loss of pitch control. In fact, the aircraft will begin to oscillate wildly in pitch. This happened to a PA30 in the UK and the crew were very lucky to survive the ensuing crash.

Loss of aircraft control in flight probably Fatal

App.: All

 

It.: Elevator Looseness

Pr.: On each preflight, check for looseness in the stabilator bearing mounting brackets by gently and slowly lifting up and pushing down on the stabilator tips listening for any "creaking sounds at the root of the stabilator at the fuselage.  There is an AD on this item as well.

Im.: Flutter and/or loss of tail in flight probably Fatal

App.: All

 

It.: Tabulator Balance

Pr.: The "all flying" tail on the Comanche is statically balanced by means of weights placed on the balance arm within the fuselage.  Over the years we have noticed many Comanches whose stabilators have not been re-balanced after maintenance or painting.  Correct balancing is critical as flutter can develop well before VNE. Various models have different methods for stabilator balance.  Those models requiring the use of a master weight (all Twins and some singles depending on serial number) should not have abrasion boots fitted to the leading edge of the stabilator as there is no Piper approved method of balancing this configuration. These stabilators should be considered to be out of balance.

In flight indications of an out of balance stabilator are:

sudden major flutter and departure of the tail surface; or (if lucky)

mild flutter, indicated by fore and aft oscillation of the control wheel, which increases in intensity with increasing speed.

Im.: Flutter and/or loss of tail in flight probably Fatal

App.: All

 

It.: Rudder gap seal

Pr.: We have noticed many of the non-counter rotating twins with the rudder gap seal either fully or partly  missing.  The seal is a critical part of the airflow kit which was introduced following the loss of significant numbers of PA30's to VMC accidents.  Beware of seals which are only present at the hinge points, check to make sure that all of the seal is in place.

Im.: Higher than expected VMC  - probably Fatal

App.: Non-CR Twin's

 

It.: Undercarriage

Pr.: Loose Hub Caps

Loose or missing screws have caused a number of these to come off and jam between the wheel and the airframe during retraction.  The result is usually an undercarriage that is stuck in the full up position.  The emergency gear extension will not work in this situation

Im.: Gear hangs in the up position.

App.: All with single fork undercarriage legs (mostly 260B's, and B and later model twins)

 

It.: Cracked Trunnions

Pr.: It is not uncommon for a crack to develop in the web of the undercarriage trunnion starting at the hole where the retract spring bracket is attached. A broken trunnion will usually result in a flat strut. If the strut does not extend on lift off, then the undercarriage cannot retract into the wing. If the left strut goes flat the squat safety switch should prevent retraction.

Whilst usually an over night problem (you arrive and find a flat strut) there have been cases of trunnions breaking on takeoff or landing, or the strut going flat and the gear jamming on retraction.

Im.: Gear Fails to retract fully or jams during retract and may not be able to be re-extended.

App.: All

 

It.: Low or no Strut Inflation

Pr.: An under-inflated strut can cause the wheel to foul on the airframe during retraction. Never operate with less than the recommended 2 inches of strut inflation (main or nose wheel).

Im.: Gear Fails to retract fully or jams during retract and may not be able to be re-extended.

App.: All

 

It.: Loose Gear Door hinges

Pr.: Very loose main gear door hinges can also cause the gear to hang in the up position.

Im.: Gear Fails to retract fully or hangs in the up position.

App.: All

 

It.: Dirty Retraction Cables

Pr.: The exposed sliding portion of the undercarriage retraction cables in the main gear wells should be kept clean but not lubricated. Greasing these cables will only attract dust and dirt which will cause the cable to seize. A badly seized cable may not be able to be extended manually.

Im.: Popping of Gear Circuit Breaker on extension or retraction.

 App.: All

 

It.: Fouling of Left Hand side Cotter Pin on retract cable to relay lever at forward end of transmission.

Pr.: There have been a number of instances where the washer and cotter pin have been assembled on the left (outer side) of the relay lever. If there is any play in the lever, the exposed end of the cotter pin can foul on the installation access hole in the vertical rib.  The result is a jamming of the gear in the up position.

Im.: Gear hangs in up position manual extension impossible.

App.: All

 

It.: Missing Torque Link Center Bolts

Pr.: On each preflight check the security of these bolts.  If they are lost, the scissors can become disconnected and the wheel may pivot randomly.  The result is usually loss of control and or gear collapse.

Im.: Loss of Ground Control Gear Collapse

App.: All

 

It.: Incorrectly assembled torque links

Pr.: After any undercarriage maintenance, check that the torque links have been reinstalled correctly so that they can fully compress. We have seen many instances of the torque links being inverted. The result is usually a broken torque link with the consequences outlined above.

Im.: Loss of Ground Control Gear Collapse

App.: All

 

It.: Missing Nose Gear Door Springs

Pr.: If the springs are missing or not connected properly the retraction arm can go over center.  The doors will retract out of sequence and jam the nose wheel in the half-retracted position.

Im.: Gear Hangs. Manual extension not possible.

App.: All twins

 

It.: Emergency Extension Handle Missing or Damaged

Pr.: An essential preflight item on all twins and 260C's is to ensure that the emergency extension handle is in its holder on the floor of the transmission well with the knob forward. It is important to keep the knob forward as the front clip is designed to retain the knob forward of the transmission in the event it becomes separated from the handle (otherwise it could roll back and foul the transmission).  You should also check the base of the handle to ensure it is not damaged or distorted as these may render it useless in an emergency.

Im.: Emergency Extension not possible

App.: All Twins and 260C's

 

It.: Missing or Disconnected Spring on Transmission for emergency extension

Pr.: At each preflight you should check that the vertical spring from the transmission disconnect to the floor is in place.  This spring lifts the forward part of the transmission upwards when the transmission is disengaged for a manual extension. If not connected, the transmission falls to the floor and jams between the relay lever and the floor.

Im.: Emergency Extension not possible

App.: All

 

It.: Insufficient over center tension on main gear torque links

Pr.: There are no mechanical down locks on the Comanche gear.  Instead it relies on the torque links being slightly (1/8") over center. It is maintained in this position by tension on the retraction cable.  If properly adjusted you should not be ale to move the center of the link upwards using light hand pressure.

Im.: Gear Collapse

App.: All

 

It.: Inoperative Throttle Micro Switch

Pr.: During preflight you should slowly open and close the throttle while listening for the actuating click of the gear warning microswitches.

Im.: Inoperable gear warning

App.: All

 

It.: Fuel System

Pr.: Mechanical Fuel Pump Vapor Lock

The plumbing of the aux pump on 180's and 250's means that while the aux pump is on, fuel is not flowing through the engine driven pump and the fuel in the engine driven pump rapidly heats.  This can result in a vapor lock in the pump and an engine stoppage when the electric pumps fails or is turned off.  It is recommended that the aux pump is checked as part of preflight and then not turned on until immediately prior to takeoff.  It should be off during the engine run ups to prove the efficacy of the engine driven pump.

Im.: Engine Failure

App.: Non Fuel Injected Aircraft

 

It.: Faulty Electric Fuel Pumps

Pr.: Most normally aspirated Comanches use a diaphragm type engine driven fuel pump.  Old age and nonuse can cause these pumps to fail. If your electric aux pumps are not working correctly (ie able to supply the correct volume and pressure) you may be faced with an engine failure.  (see TIPs Manual for advice on testing and ensuring correct pressure during maintenance).

Im.: Engine failure

App.: All but turbo'd (or formerly turbo'd)

 

It.: Blocked Fuel Injectors

Pr.: Blocked or partially blocked fuel injectors are a common problem.

Im.: Symptoms include:

A rise in fuel flow (pressure)

Rough Running, back firing

Engine surging (rpm and fuel flow fluctuations)

Reduced engine power output

A partially blocked injector can cause one cylinder to run too lean with detonation in that cylinder a possibility.

Rough Running, Low power, Possible Engine Failure

App.: Twins and fuel injected singles (some 250's, most 260's, 400's)

 

It.: Blocked Tank vents

Pr.: The fuel caps on Comanche's are not vented. A blocked vent will therefore result in the fuel cell either not feeding, or else the fuel cell being sucked flat. Tip tanks will be deformed and rubber bladders will either be lifted upwards or the cell will detach from the retainers and collapse from the top. In either case the fuel quantity indicators will read incorrectly.

Comanche vent tubes (mains, aux's and tips) have proven to be preferred homes for mud wasps.   It is a good idea to keep a length of flexible plastic tube in the aircraft and (with the fuel cap off) ensure that you can blow air into the tank.  It is a good idea to check the overflow at the same time.

Im.: Engine Failure or Fuel Exhaustion

App.: All

 

It.: Collapsed Fuel Cells

Pr.: A common problem is the collapse of a fuel cell (probably due to a blocked vent).   The capacity of the tank will be reduced as the fuel cannot push the cell up to full height.  It is therefore recommended that you keep an accurate fuel log and reconcile the quantity added against the quantity used on each fill.

Im.: Fuel exhaustion

App.: All

 

It.: Unsecured Fuel Caps

Pr.: The fuel caps on Comanche's are not vented and the fuel filler openings are located in a negative pressure area on the upper wing.  As a result a loose or missing fuel cap will result in significant quantities of fuel being lost from that tank at an extremely rapid rate. Experience has shown that a main fuel cell will be emptied overboard in less than 6 minutes if the cap is left off.

Im.: Rapid Loss of Fuel

App.: All

 

It.: Failure to drain cross feeds

Pr.: Many pilots forget to drain both cross feed lines on PA30s' and PA39s. Water can collect in the cross feed and cause a stoppage of the "good" engine if cross feed is selected in an asymmetric situation.

Im.: Engine Failure

App.: Twins

 

It.: Failed Tip Tank Solenoids

Pr.: At each preflight, you should (with the master switch on) select each tip tank and listen for the sound of the solenoid operating. This will be a solid click in the region of the main spar.

Im.: Tip or Aux Fuel not Available

App.: Nearly all Comanche's with Tip tanks

 

It.: Leaky Fuel Selector or Solenoid Valves

Pr.: Due to old age, wear or dirt in the system, the fuel selector valves may not seal perfectly.  The result can be fuel transferring between tanks or being drawn from non-selected tanks.  This appears to be more common on twins.

Im.: Unexpected Fuel Quantities

App.: All

 

It.: Failure of Alternate Air Box doors

Pr.: The original design of the alternate air door on the normally aspirated PA30's and PA39's put the door downstream of the air filter and in an area of high vibration.  A failure of the door hinge or shaft will lead to an immediate engine failure. A modification exists to move this door to a lower vibration area on the lid of the air filter. If not modified, this door should be inspected thoroughly and frequently (we have had an Australian fatality due to this).

Im.: Engine failure in flight has lead to fatalities.

App.: Normally Aspirated Twins

 

It.: Low Propeller Dome Pressure

Pr.: Gas pressure in the propeller dome is used to move the blades into the feathered position.  A faulty valve or improper maintenance  can result in dome pressures being too low to push the blades into feather.  This can be critical in an asymmetric situation. Symptoms of this problem are surging of RPMs in flight (particularly at lower power settings on descent) and a difference in pitch change times between engines. A good way to check this is to occasionally run up both engines and cycle the props together.  Any significant difference or sluggishness in cycling should be immediately followed up.

Im.: Failure to Feather.

App.: Twin

 

It.: Worn or Dirty Flap Tracks

Pr.: A worn or dirty flap track can cause the flap on that side to not retract fully after extension. This is because the flaps on all Comanches are retracted via a spring and cable arrangement which is separate for each flap. The result can be an asymmetric flap situation which will cause a significant roll moment. On twins it sometimes fees like an engine failure.  It is for this reason that touch and goes are not recommended in our Comanche's particularly the twins.  Early model aircraft with hinged (manual) flaps still have the spring retract mechanism but do not have tracks to get dirty or bind.

Im.: Asymmetric Flap Condition possible loss of control in flight

App.: All with electric flaps

 

It.: Flat or Collapsed Battery

Pr.: Comanche's are reliant on the battery to provide a "cushion of amps" during raising and lowering of the undercarriage.  A collapsed or weak battery will result in the gear circuit breaker popping and an emergency extension being required.  This can be a problem in a twin as once extended via the emergency procedure the gear cannot be quickly retracted. In the event of an engine failure you may be left with the prospect of a single engine go-around with the gear down (not likely achievable).

Im.: Various

App.: All

 

It.: Broken nose gear steering lock stops

Pr.: If the turn limits are exceed during ground handling the lock stops on the nose gear will be broken off. The resulting excessive turn can:                    Bend the bolt in the control pulley blocks situated in the forward floor section. This will result in loose cable tension and sloppy controls.                           Crack or break the rudder control arm and rudder stop brackets.                      If you see this situation on a preflight, have the aircraft inspected and repaired before further flight.

Im.: Control System Damage

App.: All

 

It.: Unexpected engagement of older style Piper Autopilots

Pr.: The early piper autopilots (Autocontrol I and II) have a roll control push pull cable to a knob on the instrument panel (usually on the lower pax side). This knob must be pulled OUT TO DISENGAGE. There have been a number of cases where this knob has been partially pushed-in, through vibration or being bumped by the co-pilot's knee. The autopilot will then engage and if the clutch is not in good condition you may not be able to override it.  The result is locked ailerons with a not so obvious cause.

Im.: Loss of Aileron Control

App.: All

 

It.: Park Brake Idiosyncrasies

Pr.: The design of the Comanche parking brake system is such that once the park rake is applied, you must release it to obtain additional braking effort.  For example, if at runup, you have applied the brake but the aircraft moves forward, you must release the park brake before operating the toe brakes or hand brake.  If you do not release the park brake, then you can push (or pull) as much as you like, but no additional braking will be attained.

Im.: Loss of Brake Control

App.: All

 

It.: Sticky Throttle, Pitch or Mixture Cables

Pr.: Beware of tight or difficult to move throttle, pitch or mixture cables, especially on twins with the long run of cable.  A seized cable can easily break and leave you with no throttle, mixture or pitch control whilst in flight.  Consider this a no-go item on your preflight.

Im.: Loss of engine control

App.: All, especially twins.

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