Definition of Primary Flight Controls? And MCF required?

[jettison valve]

Hi everyone,

I have had tough discussions concerning the definition of primary flight controls, which triggers (at least in our company) if you have to do a Maintenance Check Flight after disturbance of the system.

The best definition I could find so far is (NATO / Introduction to Flight Test Engineering, July 2005):

The flying controls of an aircraft can be regarded as being comprised of two sub-systems, namely the:

Primary flight control system (PFCS) which enables the attitude and angular rate about each axis to be controlled, and consists of the elevator, ailerons, and rudder (or their equivalents, such as the elevons on a delta aircraft, or the foreplanes with a canard configuration)

Secondary flight control system (SFCS), which permits the aerodynamic configuration to be optimised for particular flight conditions, and consists of such features as variable wing sweep (to cover the Mach number range), flaps and slats (to enhance lift and thereby improve manoeuvre capability and/or reduce takeoff and landing speeds), and airbrakes (to increase deceleration or rate of descent).

Now, we have two issues here:
1)
My colleague argues that the spoilers play a role in roll control of "modern" aircraft (A320, B747), so they should be considered PFCs.
I disagree on this as they "only" assist in roll control (B747s only beyond 45 degrees steering wheel input, if I remember correctly).
And as a gut feeling: Even simultaneous removal/installation of all A320 spoiler SCUs in my point of view does not warrant a MCF; the jobs are fairly simple, and you can test the correct installation on ground (and find spoilers left in the maintenance mode...:}).
What do you think - spoilers are primary or secondary flight control surfaces?

2)
In my company, removal/installation of the Trimable Horizontal Stabilizer Actuator triggers a MCF.
Per above definition, the THSA is not a PFC, thus there would be no MCF required. But we DO conduct MCFs after THSA replacements.
(For info on A340s in our outfit: One elevator replacement - no MCF. Both elevators replaced at the same time - MCF).
Any ideas on MCF after THSA replacement?

Have a nice sunday!
J.V.

[LOMCEVAK]

JV,

Spoilers generate a roll rate and a trimmable horizontal tailplane can generate a pitch rate (accepting that its use is to achieve zero pitch rate). Therefore, by the definitions that you have given they most certainly are primary control surfaces, a view with which I agree.

Personally, I believe that any disturbance of flaps also should generate a MCF because if one is misrigged then a significant rolling moment is generated which may mean that the nominal roll trim setting for take-off results in an aircraft that is out of roll trim at take-off.

Rgds

L

[ICT_SLB]

JV,
I'm an engineer not a pilot but I can only echo Lomcevak's opinion as far as the role of the Horizontal Stab and Spoilers in a modern aircraft. As you're well aware, in either a FBW aircraft or whenever the Autopilot is engaged on one with conventional controls, the elevator loads will be faded out by moving the Stab. Roll control can use spoilers to offset or augment elevators at angles as low as around 8 degrees - to the extent that the Autopilot can have difficulty coping with the non-linearity - much lower than in the 747.

[sevenstrokeroll]

is it activated by a wheel/stick? its primary

is it activated by a handle? its secondary


the Wrights had three flight controls, elevator, rudder and wing warping/aileron equivilent. and yes, they were activated not by a wheel.

[John Farley]

is it activated by a handle? its secondary

Far too simplistic I am afraid.

For example the Pegusus engine in the Harrier is controlled by two of your 'handles'.

There are of course many other primary flight controls not operated by a stick or wheel but I am sure this forum does not need me to list them.

[sycamore]

JV, I think you are looking at it in a simplistic manner; no aircraft is `Murphy-proof`,and especially where flying controls,be they `primary,or secondary`,whether operated by a `suitable system of cables,rods,levers,hyd.jacks or `wiggly amps` thru an autopilot. There is always the possibility that it can be wrong.
There are many of your `secondary controls` which could be described as primary,ie wing-sweep(could spoil your whole day if only one wing moved,or they moved without input.Similarly `spoilers` are used on some aircraft as `Direct Lift Control` in certain configurations. Spoilers/airbrakes are also linked in some aircraft to be an automatic Mach/IAS limiting device,opening as the airspeed reaches the limit,in a predictable manner.If they are not checked properly,and open asymmetrically/too fast/slow,it can lead to making everyones eyes water,and damage can occur. I can certainly attest to many events as a TP and MTP of having to bring an aircraft back to the `gingerbeers`,with control/rigging problems, and I`m sure JF and LCV have done the same,especially when the aircraft is just `out from maintenance`.
It is also my experience that an MCF/MTF prior to going into the shed is valuable if done by the test crew,and all the `snags/deficiencies are noted then.
Last point,take the engineers on the test flight....

[sevenstrokeroll]

I was responding to a question about airplanes...the harrier really is something a bit different...I imagine if you took the wings off you could get it up in the air for awhile.

so, vtol vehicles aside.

one must also remember at some point secondary flight controls may become primary upon failure of some primary flight controls.

thrust levers/throttles can in some planes produce pitch change or yaw change

are throttles primary flight controls? no...they are engine controls, but they can be used to fly a plane if you lose other things (pitch more so on underwing mounted engines)

good luck...I do remember reading an article which may prove interesting...the US Air Force had developed an autopilot program which could use secondary controls to return a battle damaged plane to the ground safely...using the sioux city, ia, DC10 accident as a guideline.

I also remember a Delta L1011 taking off out of KSAN, and the elevators jamming. great throttle work saved the plane

an extensive discussion of secondary flight controls becoming primary made a lasting impression on me...of course finding these article from over 30 years ago might be hard.

[ICT_SLB]

SSR,
Not hard at all - the use of throttles instead of flight controls was used by Bryce McCormick to control his DC-10 after the rear freight bay door blew out in the "Windsor Incident" - reference Chapter 8 of "Destination Disaster". Apparently this procedure was well-known to the McD instructors but only demonstrated when requested.

To bring the discussion back to current practice, there are multiple systems that now have a direct bearing on changing the aircraft's pitch, roll or yaw. Exactly which ones should be considered will depend on the detail design of a particular type - if we were still maintaining a Concorde we'd have to include the fuel transfer system!

[jettison valve]

Good evening everyone,

Thanks for your inputs, they are highly appreciated!

Regards, J.V.

[WeekendFlyer]

Hmm, difficult one! I think the risk here is getting caught up in definitions rather than focusing on what a MCF is actually for. My understanding, as someone who has worked both in maintenance and flight test of aircraft, is that the MCF is primarily there to test/prove systems that cannot be tested adequately on the ground. A similar approach is taken to test flying ; it is generally not undertaken unless the evidence needed cannot be obtained any other way or certification demands it.

Even with complex FBW aircraft, most if not all primary and secondary control surface movements can be checked thoroughly in terms of correct deflections for different flight conditions using test modes and a pitot-static test set to simulate speed and altitude. Furthermore, you would hope that maintenance work requiring a MCF would not result in a significantly higher than usual risk of the control surfaces failing to work as they should! If that were not the case, then the efficacy of the post-maintenance functional checks should be called in to question.

The only things that are not easy to test on the ground are functions that require or respond to movement (e.g. roll, pitch, yaw or longitudinal, lateral or normal accelerations), or to navigation sensor inputs. For these you have to be in flight because there is no easy way to exercise the relevant sensors and associated control loops on the ground. This would include systems such as the autopilot, auto-throttle, auto-braking and yaw damper, and also navigation functions such as FMS navigation route steering, VOR radial or ILS localiser capture, etc.

In answer to the two systems you mentioned:

Spoilers - if you have sufficient roll control and runway braking in the event of them failing, then they probably do not need a MCF. Also, there are usually several layers of redundancy in the spoiler system. An MCF would probably be required only if all the spoilers had undergone maintenance work in one go.

Tail trim actuator - a key part of the autopilot on most aircraft, and on some aircraft it becomes more critical if reduced longitudinal stability is employed in the cruise by pumping fuel aft (e.g. A340). Also, if you get a pitch-trim runaway it can be a nightmare. On the other hand, the basic trim functionality can be checked easily on the ground, with the exception of the autopilot modes that use it. On these grounds I would opt for a MCF, but focus on the operation of the autopilot vertical modes.

Hope this helps.

WF