Guys,
Is that true that in Alaska the CTOWs (take off weight) are 25 % higher than US48? Sorry for the derailment but does anybody knows the science of it?
Not necessarily, but I think a lot of their 135 operators get such certifications. For the science of it, let's take a look at the following from the C172 type certificate data sheet:
Special Ferry Flight Authorization. Flight Standards District Offices are authorized to issue Special overweight ferry flight authorizations. These airplanes are structurally satisfactory for ferry flight if maintained within the following limits: (1) Takeoff weight must not exceed 130% of the maximum weight for Normal Category; and (2) The Never Exceed Airspeed (VNE) and Maximum Structural Cruising Speed (VC) must be reduced by 30%; and (3) Forward and aft center of gravity limits may not be exceeded; and (4) Structural load factors of +2.5 g. to -1.0 g. may not be exceeded. Requirements for any additional oil should established in accordance with Advisory Circular AC23.1011-1. Increased stall speeds and reduced climb performance should be expected for the increased weights. Flight characteristics and performance at the increased weights have not been evaluated. Flight Permit for operations of overweight aircraft may be found in Advisory Circular AC21-4B.
So, as you can see, a 172 will fly at 30% over max gross, with limitations as follows:
* As noted above, Vne and Vno drop by 30%.
* Vs0 and Vs1 will increase by 14%. That's the square root of 1.3 minus 1 - Most V-speeds besides Vne/Vno will change by the square root of the ratio of the actual weight over the gross weight, and that goes for any operation - That's why if you're flying very light you'll want to use slower approach speeds to get the best landing performance, for example.
* Climb rate will be next to nothing. Climb rate is based on excess horsepower. I don't have the formula handy, but you can determine what the excess horsepower is by measuring the percent power you're using to maintain level flight at Vy, or the climb rate at maximum power. I would guess that a 172 at 30% over gross probably would max out at under 200 fpm climb.
* Cruise speed will be slower due to higher induced drag with the extra weight, increasing fuel burn per mile quite a bit.
Does this mean it's OK for you to fly over gross? NO! Ferry pilots and those pilots authorized to fly in such operations in Alaska are obviously trained on all the limitations that go along with heavy operations. Every aircraft type has different factors that lead to their certified maximum gross weight. Obviously the C172's limitation is not a structural one given what they allow above. However, at heavy weights go-arounds may not be possible past a certain point in the approach, and who knows what other limitations there will be on the plane's performance, as this post most certainly does not provide a comprehensive list. The maximum gross weight of an airplane will always be the highest it can be given FAA certification limits for structural integrity, certain required operations (such as go-arounds with flaps), and other factors. If you want to stay safe, obey the numbers.
However, I do applaud curiosity into the science behind things, so long as it doesn't tempt you to head for the edges of the envelope. Great question!