is sometimes difficult to identify unless a fuel sample is held  up against a white background in reasonable white lighting.  hangar rash incidents—dents and dings may render the  structure unairworthy. Some leading edge surfaces have  aerodynamic devices, such as stall fences, slots, or vortex  generators, and deicing equipment, such as weeping wings  and boots. If these items exist on the airplane which the pilot  intends to fly, knowledge of an acceptable level of proper  condition must be gained so that an adequate preflight  inspection may take place. Aircraft piston engines certificated for grade 80/87 run  satisfactorily on 100LL if approved as an alternate. The  reverse is not true. Fuel of a lower grade should never be  substituted for a required higher grade. Detonation will  severely damage the engine in a very short period of time.  Detonation, as the name suggests, is an explosion of the  fuel-air mixture inside the cylinder. During detonation, the  fuel/air charge (or pockets within the charge) explodes rather  than burning smoothly. Because of this explosion, the charge  exerts a much higher force on the piston and cylinder, leading  to increased noise, vibration, and cylinder head temperatures.  The violence of detonation also causes a reduction in  power. Mild detonation may increase engine wear, though  some engines can operate with mild detonation regularly.  However, severe detonation can cause engine failure in  minutes. Because of the noise that it makes, detonation is  called "engine knock" or "pinging" in cars. On metal airplanes, wingtips, fairings, and non-structural  covers may be fabricated out of thin fiberglass or plastic.  These items are frequently affected by cracks radiating from  screw holes or concentrated radiuses. Often, if any of these  items are cracked, it is practice to “stop-drill” the crack to  prevent crack progression.  [Figure 2-8]  Extra care should be  exercised to ensure that these devices are in good condition  without cracks that may render them unairworthy. Cracks  that have continued beyond a stop drilled location or any new  adjacent cracks that have formed may lead to in-flight failure.  Inspecting composite airplanes can be more challenging as  the airplanes generally have no rivets or screws to aid the pilot  in identifying spar lines and wing attach points; however,  delamination of spar to skin or other structural problems may  be identified by bubbles, fine hair-line cracks, or changes in  sound when gently tapping on the structure with a fingertip.  Anything out of place should be addressed by discussing the  issue with a properly rated aircraft mechanic. When approved for the specific airplane to be flown,  automobile gasoline is sometimes used as a substitute fuel  in certain airplanes. Its use is acceptable only when the  particular airplane has been issued a Supplemental Type  Certificate (STC) to both the airframe and engine. Jet fuel is a kerosene-based fuel for turbine engines and a  new generation of diesel-powered airplanes. Jet fuel has a  stubborn, distinctive, non-gasoline odor and is oily to the  touch. Jet fuel is clear or straw colored, although it may  appear dyed when mixed with AVGAS. Jet fuel has disastrous  consequences when introduced into AVGAS burning  reciprocating airplane engines. A reciprocating engine  operating on jet fuel may start, run, and power the airplane  for a time long enough for the airplane to become airborne  only to have the engine fail catastrophically after takeoff. While there are various formulations of aviation gasoline  (AVGAS), only three grades are conventional: 80/87,  100LL, and 100/130. 100LL is the most widely available  in the United States. AVGAS is dyed with a faint color  for grade identification: 80/87 is dyed red; 100LL is dyed  blue; and 100/130 is dyed green. All AVGAS grades have a  familiar gasoline scent and texture. 100LL with its blue dye  Jet fuel refueling trucks and dispensing equipment are marked  with JET-A placards in white characters on a black background.  Because of the dire consequences associated with misfueling,  fuel nozzles are specific to the type of fuel. AVGAS fuel filler  nozzles are straight with a constant diameter.  [Figure 2-9] However, jet fuel filler nozzles are flared at the end to prevent  insertion into AVGAS fuel tanks.  [Figure 2-10]  Using the proper, approved grade of fuel is critical for safe,  reliable engine operation. Without the proper fuel quantity,  grade, and quality, the engine(s) will likely cease to operate.  Therefore, it is imperative that the pilot visually verify that  the airplane has the correct quantity for the intended flight  plus adequate and legal reserves, as well as inspect that the  fuel is of the proper grade and that the quality of the fuel is  Figure 2-8.  Cracks radiating from screw holes that have been stop- drilled to prevent crack progression. Fuel and Oil 2-6