F-22 Raptor Flight Critical Systems
The F-22 Raptor was built with better reliability and maintainability than any military fighter in history. This helps ensure operational flexibility into the future. Maintainers were included early in the design process for the F-22 and they quickly established a strong foothold. To improve maintenance turnaround, the maintainers insisted on extensive self diagnostics and built-in testing capability for the various subsystems. As a result, virtually every piece of hardware in the aircraft either does its own health checks or reports when it has failed. There are more than 15,000 fault reports that can be made on the basic engine and airframe and another 15,000 fault reports available for the avionics. Most of these are low-level fault reports that do not result in warnings, cautions or advisories to the pilot or degrade the operation of the F-22. It was reasoned that if the airplane knew so much about itself, then that capability could be leveraged to help the maintainer and the pilot. This increased reliability and maintainability pays off in dollars, because it would require less manpower to fix the aircraft and consequently less airlift is required to support a deployed squadron. Additionally, reduced maintenance support provides the benefit of reduced life-cycle cost and the ability to operate more efficiently from prepared or dispersed operating locations.
The Vehicle Management System (VMS) provides integrated flight and propulsion control.
The VMS enables the pilot to aggressively and safely maneuver the F-22 to its maximum capabilities.
The system includes hardware, such as the control stick, throttle, rudder pedals and actuators, air data probes, accelerometers, leading edge flap drive actuators, and the primary flight control actuators. The VMS also encompasses the software that controls these devices.
The flight control software and flight control laws that underpin the VMS are tested in a specialized laboratory at LM Aero-Fort Worth, Texas.
The VMS Integration Facility, or VIF, as this lab is called, consists of an F-22 cockpit and flightworthy F-22 hardware and software. The VIF has been operational since March 1995.
The utilities and subsystems (U&S) for the F-22 includes these subsystems:
- Integrated Vehicle Subsystem Controller
- Environmental Control System
- Fire Protection
- Auxiliary Power Generation System (APGS)
- Landing Gear
- Fuel System
- Electrical System
- Arresting System
The Integrated Vehicle Subsystem Controller (IVSC) is the system responsible for aircraft integration, control and diagnostics.
The F-22 uses a totally integrated environmental control system (ECS) that provides thermal conditioning throughout the flight envelope for the pilot and the avionics.
The five basic safety critical functions the ECS must take care of include: avionics cooling; adequate air to the pilot; canopy defog; cockpit pressurization; and fire protection.
The air cycle system takes bleed air from engines (which comes in to the system at between 1,200-to-2,000 degrees Fahrenheit) and cools it down in the Primary Heat Exchanger (PHX) to approximately 400 degrees. From the heat exchanger, the air is fed into the air cycle refrigeration package (ACRP). The air must be dry, so the system also includes water extractors.
The air, when it comes out of the ACRP, is now chilled to approximately 50 degrees Fahrenheit. The flight-critical equipment, the systems that are for keeping the aircraft -flying, are cooled by this air. This air is also fed into the Normalair-Garrett-built On-Board Oxygen Generating System (OBOGS) to provide breathable oxygen to the pilot, to operate the Breathing Regulator/Anti-G (BRAG) valve on the pilot's ensemble, to provide canopy defogging, and cockpit pressurization.
Unlike other fighter aircraft, the F-22 uses liquid cooling, rather than air cooling for the mission avionics. The F-22 is breaking ground in liquid cooling and the environment in which it works. Resistance to high temperature and durability were the driving factors in the liquid cooling design. AlliedSignal is the primary supplier of the liquid cooling equipment.
The closed-loop liquid cooling system is divided into two loops, one forward and one aft. These systems use brushless, DC current motor pumps that are connected for redundancy. Polyalphaolefin (PAO) is the medium used in the liquid cooling system.
The forward loop is for cooling the Mission Critical Avionics and keep them at a comfortable (for them) 68 degrees F. The PAO passes through the Vapor Cycle System and a filter and is routed to the Avionics and then out to the wings to cool the embedded sensors.
From there, the now-warm PAO coolant enters the aft loop, which allows it to pass by the air cycle machine, which cools that system by receiving transferred heat. The PAO then is routed to the fuel tanks, where the heat is dumped. No coolant gets mixed with the fuel however, as this is a closed-loop cooling system. The fuel in the tank is only used as a heat sink.
The Thermal Management System (TMS) is used to keep the fuel cool. The Air Cooled Fuel Cooler (ACFC) takes air from the boundary layer diverter between the inlet and the aircraft's forward fuselage. Hot fuel passes through the heat exchanger and is colled. Greatly simplified, this is essentially blowing on hot soup to cool it down enough to eat it.
Fire protection is provided for the aircraft's engine bays, the Auxiliary Power Unit (APU), and for dry bays, such as the landing gear wells, the side-of-body cavities, the Linear Linkless Ammunition Handling System (LLAHS), the On-Board Inert Gas Generation System (OBIGGS), left and right ACFCs, and ECS bay.
The aircraft uses infrared and ultraviolet sensor for fire detection and Halon 1301 for fire suppression. The Halon 1301 is the only ozone-depleting chemical on the F-22, and efforts are underway to find a replacement suppressing chemical. Space provisions have already been included for this new agent up to a chemical that requires 2.5 times the volume of the Halon.
The Auxiliary Power Generation System (APGS) for the F-22 is being developed, built, and tested by Allied Signal Aerospace for Boeing. The APGS consists of an auxiliary power unit (APU), and a self-contained Stored Energy System (SES).
The APGS provides secondary aircraft power for everyday main engine ground start, aircraft ground maintenance, and in-flight emergency power for aircraft recovery. The APGS uses the G-250 APU, a 450 hp turbine engine that utilizes state-of-the-art materials and design resulting in the highest power density APU in the industry (horsepower-to-weight).
The F-22 utilizes tricycle landing gear, with the standard two main gears (each with a single tire) and a single-wheel, steerable nose landing gear assembly. The nose gear retracts forward, and main gear retracts outward.
The landing gear assemblies utilize AirMet 100, which provides greater strength and corrosion protection and are made by Menasco. The main gear uses a dual-piston design and are sized not to withstand a collapsed gear or flat tire landing.
The aircraft's AlliedSignal-made carbon brakes are always in anti-skid mode, which means the pilot has one less thing to remember to activate. The pilot applies pressure on the brakes by using the rudder pedals, but only after the F-22's weight-on-wheels sensor engages upon landing.
The nosewheel is a direct drive system, that is hydraulic force is applied to the nosewheel pivot to turn it. The nose gear is mechanically driven to align itself correctly before retraction.
As a safety precaution, the nosewheel clamshell doors and the lower inboard landing gear doors are physically linked to the landing gear itself. If an emergency blowdown is required, the doors will open when the gear comes down. Also, the gear down and locked indicators in the cockpit are battery operated, so if all other systems malfunction, the pilot still has a way of knowing whether his landing gear is down.
The tires on the F-22 are Michelin Air-X steel belted radials. Goodyear Bias-ply tires were also qualified for the aircraft.
There are eight fuel tanks on the F-22, including one (designated F-1) in the forward fuselage behind the pilot's ejection seat. The others are located in the fuselage and the wings. The F-22 runs on JP-8, a naphthalene-based fuel with a relatively high flash point.
The F-22 has single-point ground fueling, and it can be refueled without the need for ground power. It also has a Xar-built air refueling receptacle on the top side of the aircraft in the mid fuselage directly behind the cockpit. It is covered by two butterfly doors that have integral low-voltage lights to aid in night refueling.
The F-22 also has an On-Board Inert Gas Generation System (OBIGGS) that inerts the fuel tanks as the fuel is depleted. Fuel in itself is not as explosive as the fumes are. By filling the tanks with inert nitrogen as the fuel is used, the fumes are suppressed, and the chance of explosion, such would occur if the fuel tanks were hit by gunfire, is nearly eliminated.
The F-22 uses a Smiths Industries 270 volt, direct current (DC) electrical system. It uses two 65 kilowatt generators. The hydraulic system includes four 72 gallon-per-minute pumps and two independent 4,000 psi systems.
The F-22 has an arresting hook in an enclosed fairing between the engines on the underside of the aircraft. This hook is deployed in an emergency to stop the aircraft by means of hooking on to a wire strung out across the end of a runway. These barrier engagements work very similar to the arresting gear of an aircraft carrier.
While the F-22 has an arresting hook, it cannot land on an aircraft carrier, as the F-22 does not have the heavier structure necessary to withstand the stresses of a carrier landing. The shape of the arresting hook is not compatible with low observable design, and that is why the fairing and doors are required.
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