CHAPTER 12 Airworthiness.............................................................................
The airworthiness of an aircraft is concerned with the standards of safety incorporated in all aspects
of its construction. These range from structural strength to the provision of certain safeguards in the
eventofcrashlandings,andincludedesignrequirementsrelatingtoaerodynamics,performance,and
electricalandhydraulicsystems.Theselectionofminimumstandardsofsafetyislargelytheconcern
of “national and international” airworthiness authorities who prepare handbooks of official require-
ments.Thehandbooksincludeoperationalrequirements,minimumsafetyrequirements,recommended
practicesanddesigndata,andsoon.
Inthischapter,weshallconcentrateonthestructuralaspectsofairworthinesswhichdepend,chiefly,
onthestrengthandstiffnessoftheaircraft.Stiffnessproblemsmaybeconvenientlygroupedunderthe
headingaeroelasticity.Strengthproblemsarise,aswehaveseen,fromgroundandairloads,andtheir
magnitudesdependontheselectionofmaneuveringandotherconditionsapplicabletotheoperational
requirementsofaparticularaircraft.
12.1 FactorsofSafety-FlightEnvelope.................................................................
Thecontrolofweightinaircraftdesignisofextremeimportance.Increasesinweightrequirestronger
structures to support them, which in turn lead to further increases in weight and so on. Excesses of
structuralweightmeanlesseramountsofpayload,therebyaffectingtheeconomicviabilityoftheair-
craft.Theaircraftdesigneristhereforeconstantlyseekingtoparehisaircraft’sweighttotheminimum
compatiblewithsafety.However,toensuregeneralminimumstandardsofstrengthandsafety,airwor-
thiness regulations lay down several factors which the primary structure of the aircraft must satisfy.
These are thelimit load, which is the maximum load that the aircraft is expected to experience in
normaloperation;theproofload,whichistheproductofthelimitloadandtheprooffactor(1.0–1.25);
andtheultimateload,whichistheproductofthelimitloadandtheultimatefactor(usually1.5).The
aircraft’s structure must withstand the proof load without detrimental distortion and should not fail
untiltheultimateloadhasbeenachieved.Theproofandultimatefactorsmayberegardedasfactorsof
safetyandprovideforvariouscontingenciesanduncertainties,whicharediscussedingreaterdetailin
Section12.2.
The basic strength and flight performance limits for a particular aircraft are selected by the
airworthinessauthoritiesandarecontainedintheflightenvelopeorV−ndiagramshowninFig.12.1.
Copyright©2010,T.H.G.Megson. PublishedbyElsevierLtd. Allrightsreserved.
DOI:10.1016/B978-1-85617-932-4.00012-9 373