The four forces acting on an aircraft in flight are lift, weight, thrust, and drag. Load factor on a small unmanned aircraft rises most when the pilot makes a steep, fast turn or an abrupt maneuver. On a standard day at sea level, atmospheric pressure and temperature are 29.92 inches of mercury and 15 °C, which is 59 °F.
Lift, weight, thrust, and drag
Lift is the aerodynamic force that acts against the aircraft's weight. On a multirotor it comes from the rotors moving air. On a fixed-wing aircraft it comes from the wing. Weight is the downward force of gravity, the aircraft and everything attached to it. Thrust is the force that moves the aircraft forward or, on a multirotor, the force the rotors apply along the direction you command. Drag is the resistance of the air to that motion.
Those four names are the list. Wind, rain, sun, and temperature are weather, not the four forces. Lift, speed, altitude, and heading mix one real force with three things that are not forces at all. Speed is a result of the forces. Altitude is a position. Heading is a direction. If the stem asks for the four forces acting on an aircraft in flight, the answer is lift, weight, thrust, and drag.
Section 107.73 includes small unmanned aircraft loading and determining performance among the knowledge areas on the initial test and in recurrent training. The four forces are the vocabulary those tasks use. Where the aircraft's weight sits, and how that changes when you add a payload, is the center of gravity topic. How thin air reduces the lift and thrust the rotors can produce is density altitude.
Load factor rises in a bank
Load factor is the load the structure supports compared with the aircraft's weight. In a steep, fast turn, or in an abrupt pitch or stop, that load goes up. The frame and the motors have to produce more force than they do in gentle straight flight. A slow descent straight down does not increase load factor the way a steep bank does. Powering the aircraft off does not increase it either. The maneuver that raises it is the aggressive one: a steep turn or a sudden control input.
The bank-angle table, including the values at 30, 45, and 60 degrees, is drawn on the load factor page from FAA-CT-8080-2H Figure 2. This page's job is the direction of the change. Load factor increases most in the steep, fast turn or the abrupt maneuver. Smooth inputs keep it lower. A small airframe can be overstressed by a yank that a larger aircraft would shrug off, and the extra force spends battery because the motors are working harder.
The standard day the performance numbers start from
A standard day at sea level is the baseline those performance ideas use. Atmospheric pressure is 29.92 inches of mercury. Temperature is 15 °C. The same temperature expressed in Fahrenheit is 59 °F. The question asks for the pair closest to standard, and the answer is 29.92 inHg and 15 °C. The explanation states both temperature units because 15 °C and 59 °F are the same standard temperature.
The trap in the choices is to attach 59 °F to a nighttime-only rule, or to offer 28.00 inHg and 20 °C. Standard sea-level pressure is 29.92, not 28.00. The standard temperature is 15 °C, and it is not limited to night. You do not need a second set of standard numbers for this item. Pressure and temperature at sea level on a standard day are 29.92 inHg and 15 °C (59 °F).
When the real day is hotter than 15 °C, or the field is well above sea level, or the air is humid, density altitude rises and rotor performance falls. That is why the standard-day pair shows up in a performance lesson even though it looks like a weather fact. It is the reference the other performance answers move away from.
A turn on a standard morning
You launch from a sea-level site on a morning that matches the standard values, 29.92 inHg and 15 °C. The aircraft in steady cruise is subject to lift, weight, thrust, and drag. You need a tight orbit around a tower, so you bank steeply and you pull the turn harder than a gentle arc. Load factor rises because of that bank and that abruptness. It would not have risen the same way if you had descended slowly in a straight line or if you had simply shut the motors down. After the orbit you go back to smooth inputs. The standard-day numbers described the air you launched in. They did not authorize the steep bank, and they do not change which four forces are acting.
The trap
The force question is missed by listing weather or by listing speed, altitude, and heading. The load-factor question is missed by picking a gentle descent or a power-off condition instead of the steep, fast turn. The standard-day question is missed by limiting 59 °F to night, or by substituting 28.00 inHg and 20 °C. Lift, weight, thrust, and drag are the forces. Load factor rises in the aggressive maneuver. The standard sea-level day is 29.92 inHg and 15 °C (59 °F).
Practice the performance items on the free practice test. Loading and performance are taught in the course.