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WnB GC limits considerations for speeds, flaps and gear


I am just wondering if one can fail WnB/GC limits under 1/ specific speeds & heights (don’t think so as included in performance tables and flight envelope operating ranges) or 2/ surface configuration (mainly flaps & gear) ?

I come across a glider last weekend where WnB/GC limits from PoH has been overridden by new placard limits (from engineer inspection report), I was told it has to do with GC limits on specific gear & flap configuration, I find that explanation very odd (the new limits were far more conservative than POH)

ESSEX, United Kingdom

Our aircraft (P210) has a rearward CG shift when retracting the landing gear. This is already factored in the limitations envelope. So, in theory but illegally, you could operate with gear extended and CG aft of the limit and you might be in compliance with all of the other required handling criteria, as there is some margin for retraction. If you then retracted the gear then stability and control criteria would be exceeded.
OTOH, our aircraft does have a different CG “limit” for T/O and LDG, but that is due to expected CG shift as fuel is used in flight: if T/O CG is aft of the “restricted” limit then you have to check that fuel usage during flight will not shift CG aft of the “absolute” limit for landing.

I do not believe aircraft will however exhibit different CG limitations for gear retracted and extended: they just take the most conservative one, or else you would be able to load it incorrectly simply by retracting the gear, for example. I am sure a similar thing occurs with flaps.

This may be different on aircraft that have a means of controlling CG in flight (other than by moving SLF * !). Very fast aircraft do have different CG envelopes based on speed due to compressibility effects, but some also have a means of adjusting CG (trim fuel tanks…)

* Picture the flight attendant asking passengers to move forward in the cabin in preparation for gear retraction

Last Edited by Antonio at 29 May 09:52
LESB, Spain

Gear and flap, move the lift and drag distribution. Flaps move the lift distribution towards the root (assuming they are not full span) affecting roll stability and stall characteristics. The centre of lift often moves with flap deployment. All these things affect the stability and handling of the aircraft.

I don’t think there is any technical reason stopping the OEM establishing different balance limits with an aircraft in a different configuration, at least in GA. However how would you actually make use of it? In case of the glider I suspect they found a problem which was worse in a specific configuration, as a result of some test flying.

Rather than placarding “undercarriage must remain retracted at all times” a new CG limit was created.

For example flap deployment at rear CG, might have made the spin characteristics outside a specific certification limit.

Last Edited by Ted at 30 May 09:55

Ibra wrote:

I find that explanation very odd

Good for you, for a type certified aircraft, it’s odd. The W&B limits for a type certified aircraft will always be specified on the type certificate for that aircraft. The only legal way to change these, or any other information on the type certificate, is by the issuance of a supplemental type certificate, for which there is a process. I have issued STC’s which include W&B limitation changes. If W&B (or any other limitation is changed by STC, there will be an approved flight manual supplement (and possibly a placard) to describe the changed limitation. The FMS will state the approval number.

If there were an authoritative reason that a W&B limitation be restricted, that will either be conveyed by an STC, which will be associated with another change, which dictated the need for a W&B limitation reduction, or by AD.

An example of this has been my issuing an STC for a Cessna 185 amphibian, for which approval of undersized floats was requested. I approved the float installation, though with a gross weight limitation appropriate to the reduced bouyancy of the smaller floats. Sadly/ironically, the new pilot disregarded the FMS I drafted, and the placard which I installed, and sunk the plane for being overweight – oh well… (oh, and by the way, the insurance did not pay for it, he had exceeded limitations = C of A invalid). Yeah, and at the weight he sank, he would still have been overweight for a 185 floatplane without my weight limitation reduction!

C of G range reductions may be approved as a part of an overweight ferry flight authorization.

Ibra wrote:
WnB/GC limits from PoH has been overridden by new placard limits (from engineer inspection report)

If an engineer determines/suspects that a limitation change is required, that person would have to report the condition as a defect to the authority, and let them act upon it. The Engineer has no authority to redefine limitations, other than by seeking an STC. It would be unheard of to apply an STC to an aircraft to simply reduce W&B limitations, as the installation of an STC is not mandatory. It is an AD which his mandatory, so that would be the only way to reduce limitations, and only the authority issues AD’s.

So, if you’re curious, ask the Engineer under what approval the limitation was issued – STC? AD? Note that for non certified aircraft, W&B rules are different, but there still are rules…

Home runway, in central Ontario, Canada

Ibra, do you remember what type of glider it was?

In the BGA CAMO, gliders are reweighed after large mode/repair and every 8 years. For minor stuff you do the math or use engineering judgement.
Repairs and time tends to increase the mass and move the empty CoG rearwards. Resulting respectively in lower max and min weight for the pilot.
Never heard of different flaps or gear.

You can have different cokpit weight limit and Airspeed/g limit with different wing tip / winglet (some gliders can put different tips to be 15 or 18m span)
Speaking of gear the only possibility I see has to do with the tail wheel. As it is beneficial in weak (UK) soaring condition to fly with the CoG close to the aft limit, heavy pilots might replace a plastic or aluminium tail wheel with a brass tail wheel. That would require a reweight and re-placarding.

More modern gliders have water tanks in their fin and the proper documentation in the AFM which make CoG adjustment easier.

Nympsfield, United Kingdom

The glider was an ASW15A maintained under BGA CAMO, the new limits was -18kg more conservative than POH numbers on its useful load

ESSEX, United Kingdom
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