abushey123 wrote:Hey guys/gals. I'm new to the world of 170s. Recently scooped up a 1953 170b for not a lot of money but it is looking a little tender in a few spots. The area in question is the leading edge on the horizontal. Currently there is a dent in the leading edge about the size of a golf ball just outboard of where the spar ends. It is pushed in just so the bottom skin has a small wave in it. Curious if anyone has the published limits or knows if this is acceptable. I have had a few mechanics look at it. Half say its fine, half say its not. Just looking to clear it up. If anyone has a pdf of the 100 series maintenance manual that would be a wonderful help!!
A Picture would certainly be helpful. The applicable “maintenance manual” for the 170 airplane is known as the “Cessna 100 Series Service Manual, 1962 and Prior”, which has a “Structural Repair” section containing some guidance of a general nature. Another useful document is the Cessna Structural Repair Manual for later aircraft (172, 182, etc.)
Generally speaking there are three descriptions of damage-levels: Negligible, Repairable, and Replacement. Those are further defined in the opening paragraphs of each section pertaining to each area of the airframe.
The Cessna Structural Repair Manual states that the horizontal stabilizer damage should be defined using the fuselage descriptions…. and in that chapter it is said:
“ Negligible Damage
A. Any smooth dents in the fuselage skin that are free from cracks, abrasions, and sharp corners, and which are not stress wrinkles and do not interfere with any internal structure or mechanism, may be considered as negligible damage. In areas of low stress intensity, cracks, deep scratches, or deep, sharp dents - which after trimming or stop-drilling can be enclosed by a two-inch circle - can be considered negligible if the damaged area is at least one diameter of the enclosing circle away from all existing rivet lines and material edges. Stop drilling is considered a temporary repair and a permanent repair must be made as soon as practical…..”
Unfortunately, no mention is made in the fuselage chapter regarding “leading edges”. However in the chapter for wings it is stated: “
“ Negligible damage: Any smooth dents in the wing leading edge skin that are not more than 0.030 inch (0.76 mm) below contour and circumscribable with not more than a 1.5 inch (38 mm) diameter circle that has no evidence of skin tears, cracks, or skin penetrations - which are not stress wrinkles and do not interfere with internal structure - constitute negligible damage.
However, because of the critical nature of the wing leading edge, this cosmetic repair should be completed…..”
Of equal or greater concern is the hidden damage which may exist depending upon the cause of the damage you describe (and why a pic would help.)
Think of this… if the dent is caused by a rock that flew up during landing on a rough strip….the damage is likely limited to the local area of the stabilizer leading edge. BUT…if the dent is caused by a “strike” such as hitting a runway light fixture during a “runway excursion” … (ground loop or temporary loss of directional control, etc.)…. then it should be recognized that a significant dent in the leading edge is not a “cosmetic” problem…it can be a STRUCTURAL problem to the stabilizer, especially to the rear spar attachment area.
The “strike” which was sufficient to cause the dent in such a scenario will have imparted a MUCH GREATER FORCE to the interior structure …much like a “lever” can increase the forces applied to an object. (A similar corollary can be drawn to what a casual observer might think is only minor damage to a wingtip fairing that struck the hangar upright…. But the subsequent Force Applied to the rear spar at the fuselage attachment area can be MANY TIMES GREATER than that which made only negligible damage to the wingtip fairing.
Since you wrote the damage was in the outer section of the horiz-stab leading edge… it’s possible quite a shock was applied to the stabilizer structure itself and the attachment to the fuselage. A crack there can have serious consequences which may not be immediately apparent.
Also, consider that the leading edge of any flying-surface such as a wing or stabilizer can affect the airflow of the lifting surfaces aft of the damage. In the description you give of the dent… it is on the lower surface….”pushed in so the bottom skin has a small wave in it”. Again, a pic would help one to determine if this is negligible or repairable or a candidate for replacement… but remember the horizontal stabilizer acts as an “upside-down wing”… It creates a “downward lifting force” to control the pitching moment of the airplane about the center-of-lift at the wing. Any disruption of air-flow caused by damage to a leading edge can adversely affect flying and handling characteristics…Perhaps not in most normal operations…but may have more serious controllability issues at the extremes such as in stall/spin/slow-flight/takeoff/landing regimes.
You’ve done the right thing consulting with your A&P/IA (and seeking help here where there are people far better and more experienced than I to offer help….but while I’m at it… WELCOME to the 170 “type club”…. contact our headquarters and JOIN!) . I’d suggest you give additional consideration to how that damage occurred and what hidden damage possibility might exist. Not all A&Ps have experience in structural repairs…. a repairman with structural repair experience might be best suited to help you make a determination.
Hope this helps.