Smoke On Go

Load factor limits: How much g is your aircraft certified for?

Let’s take a look at what’s meant by a load factor, how it’s calculated, and how it differs from g.

The weight of an aircraft is created by the force of the earth’s gravitational attraction. No matter what attitude the aircraft happens to be in, the weight vector will always act vertically down towards the centre of the earth. On the other hand, the lift generated by an aircraft’s wings always has a vector that acts at right angles to it.

In straight and level flight, the weight of an aircraft is exactly balanced by the lift being generated by the wings and perhaps some other parts that make up its structure.

We may say, therefore, that the load of the aircraft is being balanced by the loading of the wing. The ratio of the loading on the wing to the load of the aircraft gives the load factor. Therefore, an aircraft in straight and level flight will have a load factor of 1.

The load factor or the g an aircraft is being subjected to are one and the same. Values of g are also dimensionless ratios and are calculated by dividing the acceleration due to gravity into any acceleration that the aircraft is being subjected to while manoeuvring.

The ratio of the loading on the wing to the load of the aircraft gives the load factor.

A couple of examples

If an aircraft is in a 60° banked level turn, its lift vector would be tilted at an angle of 60° from the vertical. This lift vector can be split into two component vectors. One is the horizontal vector that provides the centripetal force to turn the aircraft, and the other the vertical vector that acts to balance the weight of the aircraft.

A vector diagram would immediately show that the lift vector would need to be increased by exactly double what it was in order for its vertical component to balance the lift. While the load of the aircraft has remained constant, the loading on the wing is twice what it was in straight and level flight. The load factor would now be 2, and the aircraft would be in a 2 g turn.

HAVE YOU READ: Minimum control speeds for light twins – Part 1

Aircraft limitations

Contained in every aircraft’s Pilot’s Operating Handbook is a comprehensive list of all operating limitations. Included are g limitations that pertain to the various categories in which the aircraft may be operated.

All civilian aircraft manufactured and certified in the USA have limit load factors that are based on the maximum amount of g that the airframe must be able to withstand before deformation, severe damage or even disintegration becomes a possibility.

These limitations are :

  • plus 6 g for aircraft in the aerobatic category
  • plus 4.4 g for utility category aircraft
  • plus 3.8 g for aircraft in the normal category
  • airliners are limited to 2.5 g with flaps retracted and 2 g with flaps extended.

The safety factor

Added to these load limit factors is a safety factor of 50 % that is required above the load factor limits. To some extent, this provides for unintended loads that are higher than those expected in normal, reasonable operations. These values are known as ultimate load factors.  

This means, for example, that a normal category aircraft should be able to withstand up to, but not beyond, a load factor of 3.8 g x 1.5 = 5.7 g without failure. However, it is accepted that some parts of the aircraft may twist or bend under these loads and that some structural damage could occur.

In a 30° level turn, a mere 1.15 g is pulled, but this value rises exponentially as the bank increases. At 45°, it will be 1.41 g and at 60°, it’s 2 g. Increasing the bank angle to 75° will require a huge 3.86 g to keep the aircraft level. This will also exceed the limitation for a normal category aircraft.

Angles of bank for steep turns that are between 45 and 60° are therefore well within the limit envelopes for aircraft in all categories, amazingly, even airliners as big as the Boeing 747.

Aircraft designed under the aforementioned “category system” are readily identified by a placard on either the instrument panel, or on a cockpit side panel. The category of operation and maximum safe g limit is displayed there.

Certain aircraft are certified in two categories

The Piper Cherokee 140 is a popular training aircraft that may be operated in both the normal category (+3.8 g) and the utility category (+4.4 g). In order to fulfil the utility category limitations, only the two front seats may be occupied and the fuel tanks must be filled to a maximum of 50% capacity. There’s a metallic step located in the fuel tank filler port that indicates this fuel limit. Positive spins are then allowed in the aircraft, as well as chandelles and lazy eights.

However, if the aircraft is loaded with, for example, a full fuel tank plus four people and baggage, it must be operated only in the normal category. In this case, spinning, lazy eights and chandelles are prohibited.

Gentle aerobatics have been performed in certain utility category aircraft The late Bob Hoover was famous for his aerobatic displays in the Aero Commander Shrike. This aircraft started life as a utility category aircraft and therefore had a load limit factor of 4.4 g. It was devoid of all unnecessary, weighty items and carried minimal fuel during the displays. In his very skilled hands, loops and barrel rolls could be accomplished − pulling only 3.5 to 4 g. I know this because I once spent time with him and asked about this.

Straight rolls need to be performed below the maximum allowable speed for full aileron deflection. For less experienced pilots, the most important consideration is that the pilot NEVER becomes disorientated in any manoeuvre and “loses” the way. This will certainly overstress the aircraft during the recovery.

Other vital aspects are that the aircraft must have the all-round control authority in pitch, roll and yaw to actually perform the required manoeuvres. In addition, it should not be prone to excessive speed build-up when the nose is pointing vertically downwards.

Older, lighter and smaller vintage aircraft, many of which are still around, such as the Piper Rag-wings and the Cessna 140, were built according to the requirements before the advent of operational categories.

Since the speeds that these aircraft are able to achieve are very low, it is not possible to pull much g in them in any case. Maximum load factor limits are nevertheless published for most of these aircraft. The Cessna 140, an aircraft that I fly on a regular basis, is limited to a very generous 4.6 g.

Have fun out there, but know both your own and your aircraft’s limitations!