The International Cessna^® 170 Association

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!!

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.

After talking with the previous owner, it sounds the dent was caused by a rock on takeoff. I have linked a photo from my google drive for further review. You can see the push out of the bottom skin off to the left of the dent. https://photos.app.goo.gl/DRGdHL3GuK6bC7eJA


I’ve paid my membership for the year happy to be part of the club. Now I just need to get this bird flying!!

Abel, I've just upgraded your forum privileges to reflect TIC170A membership. You will now see a lot more information in our maintenance library and now have the privilege's to post pictures directly at the forum.

Welcome

Abel… as long as that snow and ice remains on the airplane….that little Dent is “negligible”.

Welcome Abel glad you made it here to the forums and thks for joining the association.
Couple of things to consider to deal with your leading edge damage. Most Structural repair manuals (SRM's) are structured where the beginning starts with definitions, part identification, damage category's, after which comes allowable damage depending on the component and location, and finally repair. In the allowable section will be damage which allows an airplane to fly without immediate repair by specifying damage limits. This is just the basics but the theme is the same regardless of the structure. Typically damage is a combination of damage types and each damage type must be assessed on its own merit. An object may have caused the metal to deflect past its yield point and deform the metal into a dent, if that object is rough it may also have caused a loss of material ie a gouge in the center of a dent. The depth of a dent relative to the material thickness and in relation (distance from )to structural elements also must be considered. Blah blah blah. Whenever we deal with structural damage and take a picture of it we always put a ruler in the pic, typical right beside the damage that allows a viewer to quickly assess the damage size. Specifically with dents there's a edge distance from the edge of the dent to the nearest fastener and an inspection to ensure with skin damage that the ie rivet is not being pulled. You could measure the depth by laying a ruler flat along the skin to project the surface and measuring the depth. For crack prevention, there's an depth to width ratio for dents limits where the greater the depth, you need a larger area of the dent ,in other words a smooth dent with spread out is unlikely to crack verses a deep dent in a small spot.
So I'd say as long as nothing is cracked, inside and out, no pulled rivets, no sharp edges, the dent not encroaching on internal structure I would clean the entire leading edge, smooth any gouges, fill in the void and recontour the leading edge with filler in the damage area, then prep /paint the entire leading edge as it looks like it could use some love. But some better pics as mentioned would be better for a total assessment from a distance.

Here is the pic from the link:

Here is a even better picture of the dent.

and another.

and the last.

If the previous owner claimed that was caused by a “rock on takeoff”…. I believe he needs to consider the differences between a “rock” and a “boulder”.
<edit> Might have been a good opportunity for a “rejected takeoff” and presents incredulity to the claim of how it occurred, IMO.

I’d definitely have that re-evaluated including an inspection of the interior and attachment at the fuselage for hidden damage. THAT is not “negligible”.

Ok so as I mentioned previously, if the dent goes beyond some rivets, that would be a problem on any aircraft. It appears the damage goes beyond just skin damage that's another damage criteria crossed necessitating a further investigation of the damage extent and likely repair.
Nothing wrong with George's suggestion to look at the attach points of the stab for damage, its probably coming off anyway. While its off, you might want to replace the tail wheel yoke hardware, looks like they have seen better days.
Looks like you need some sheet metal talent.
You might want to pretty up the rudder while you're at it and you might want to read up on the many posts on tail wheel springs.
Finding a replacement stab might be a quicker, cheaper alternative, good sheet metal is getting harder to find.

Yes, I'd have to look at this damage little closer and likely at least have to repair the leading edge skin.
And if you are wondering what this cable outlet is for, your plane probably had wing leveler installed at one time. The wing levelers of the time used the rudder to level the wings and that was controlled via extra cables to the rudder.

Often you will find L-19 parts that match those on the C-170. They are often more readily available and significantly cheaper than C-170 parts. Here's an example. $20 and free shipping -- that's hard to beat! Unfortunately, the stab leading edge on the L-19 has a hole in it for the dipole antenna mount. Some time ago I visited Faeth Aircraft and they had a couple of 170 horizontal stabilizers in various states of disrepair. Might be a source of parts if you can't find NOS parts. Here's a picture of one they had.

I’m wondering if we weren’t being punked a little bit… since the first pic showed only a tiny dent with snow…and then after time for comments…the real picture was revealed.