Incidenti aerei Cold Case, le analisi FAA, JAA, NTSB e EASA su "ullage"
Serbatoi esplosivi e vapori di benzina: uno scenario allarmante!
E' forse anche il caso dell'Airbus 320 Egyptair MS-804 dello scorso 19 maggio 2016? Gli investigatori cosa sanno a riguardo? Quanti altri Cold case potrebbero essere stati "innescati" da "fuel tank inerting"?
Il dizionario alla voce "ullage" riferisce :quantità (f.) di liquido mancante (in una botte ecc.); calo (m.); contenuto (m.) effettivo (di un contenitore per liquidi non pieno), nel mondo aeronautico l'utilizzo di tale termine è abbastanza recente e la EASA a riguarda sostiene: "Ullage, or Ullage Space. The volume within the tank not occupied by liquid fuel at the time interval under evaluation".
In questi anni sarebbero stati adottati sistemi "ullage test" (EPA-USA tra gli altri) al fine di controllare la tenuta dei serbatoi in genere e, probabilmente anche quelli installati sulle flotte aeree.
L'EASA nel 2010 ha inquadrato il "rischio esplosione" nel Safety Information Bulletin SIB No.: 2010-10 Issued: 31 March 2010 Subject: Fuel Tank Safety – Flammability Reduction System (FRS) for High Flammability Exposure Fuel Tanks – Production Cut-in Applicability: New production Airbus A318, A319, A320, A321, A330-200 and A340 and Boeing 737, 767 and 777 aeroplanes, fitted with Centre Wing Tank.
Lo ha ribadito anche nel novembre 2014 nel : EASA Safety Information Bulletin SIB No.: 2010-10R1 Issued: 28 November 2014 Subject: Fuel Tank Safety – Flammability Reduction System (FRS) for High Flammability Exposure Fuel Tanks Ref. Publications: EASA Executive Director (ED) Decision 2014/024/R dated 21 July 2014, and the related Explanatory Note.
Perchè anche l'aviazione commerciale e quella militare hanno ritenuto importante se non rilevante argomentare su questa questione ad oltre 100 anni dal primo volo?
Se "ullage", identifica lo spazio vuoto in un contenitore di liquidi e/o un serbatoio di carburante - quelli ad esempio ubicati nelle ali dei velivoli e/o in contenitori alloggiati nella parte sottostante delle fusoliere - "ullage" è divenuto un termine/parola inglese derivata dalla terminologia francese ouillage, ovvero al rabbocco.
Al lessico "ullage" ci si richiama come ad una ipotetica causa, dalla quale scaturisce l'innesco di una sorta di miscela infiammabile tra aria-carburante e conseguente esplosione dei vapori di carburante esistenti nel serbatoio.
La materia è nota anche in ambito esterno all'universo del trasporto aereo. Lo stesso Ministero dell'Interno - Centro Studi ed esperienze, ha - in uno studio di 61 pagine - Studio sui sistemi di sicurezza per effettuazione a ciclo chiuso e recupero vapori nei serbatoi interrati destinati al contenimento dei carburanti liquidi di Categoria A (liquidi i cui vapori possono dar luogo a scoppio) - ha analizzato il fenomeno e i criteri di controllo e gestione.
L'EASA - European Aviation Safety Agency - in data 17 luglio 2008 aveva proposta uan NOTICE OF PROPOSED AMENDMENT (NPA) NO 200819 in relazione alla “Fuel tank flammability reduction”.
Alla pagina 5 delle 26 si legge:
"Existing Design Principles
Contributing factors to these accidents were the design and certification concept that fuel tank explosions could be prevented solely by precluding all ignition sources. This is important in relation to the design of aircraft that have heat sources underneath the centre tanks. These features result in the tank ullage (the volume within the fuel tank not occupied by liquid fuel) being flammable to the extent that very small energy levels can ignite fuel vapours and to the extent that the overall risk is increased because the tank ullage remains in the flammable range for a significant proportion of the aircraft operational time. Eliminating all ignition sources from tanks that have a high flammability exposure may not be practically achievable.
The NTSB recommendations did include making improvements to and maintaining the safety of fuel tank designs. This could be achieved by reducing the probability of creating an ignition source within fuel tanks and also minimizing the development of flammable vapours in heated centre tanks.
Commercial transport aeroplane fuel tank safety requirements have remained relatively unchanged throughout the evolution of pistonpowered aircraft and later into the jet age. The fundamental premise for ensuring protection from fuel tank explosions has involved establishing that the design did not develop a condition that would result in an ignition within the fuel tank ullage space (i.e. ignition prevention) as well as result in a heated surface that would cause autoignition of the fuel vapour. A basic assumption in this approach has been that the fuel tank could contain flammable vapours under a wide range of conditions even though it was recognized that there were periods of time in which the vapour space would either be too lean or too rich to support combustion. The use of Jet A/A1 and Jet B fuels and mixtures of both fuels in early jet operations made it difficult to predict when and where the tanks would be flammable. An accident involving a lighting strike to the wing of a Boeing 707 in the early 1960’s which resulted in catastrophic wing failure underscored the importance of protecting the fuel system from the direct (e.g. stroke penetration) and indirect (e.g. electrically induced) effects of lightning. This accident resulted in additional fuel system
lightning protection/fuel system requirements being added to FAR part 25 in the mid 1960’s.
The focus remained on prevention of the ignition of vapour by preventing the lightning caused spark from occurring. The reliance on spark or hot surface prevention as the principal safety strategy has been
largely a consequence of the stateoftheart in fuel tank and aviation system technology.
Previous attempts to develop commercially viable systems or features which would reduce or eliminate either aspects of the “fire triangle” (i.e. fuel, oxygen, ignition) such as fuel tank inerting or ullage space vapour “scrubbing” (i.e. vapour sweeping in order to prevent the accumulation of sufficient concentrations of fuel vapour to become flammable) proved to be unrealistic due to the weight of the systems, poor reliability, or undesirable secondary effects such as unacceptable atmospheric pollution." (per le restanti note alleghiamo il PDF EASA).
Sono stati inquadrati i seguenti ambiti:
- Developments and research into flammability reduction
- Existing Requirements (JAA-FAA-EASA).
Altri documenti rimandano al "Regulatory Impact Assessment For the Introduction of a Flammability Reduction System".
Le considerazioni economiche e dei costi relativi rimandano ad un paragrafo specifico "Economic" dove sono stati stimati i costi di "manutenzione e adeguamento" degli interventi al fine di fronteggiare tali eventi.
"Considering that other regulatory initiatives, including the one led by FAA, have a much larger impact on the large transport aeroplane fleet, no detailed quantitative economic assessment is provided in this NPA. The FAA NPRM 4 provides information on the cost associated to mandating FRM systems for future TC, aircraft in production and retrofit to the existing fleet. In a similar manner the EASA RIA 5 produced in 2004 provides comparable information. However the data included into this RIA needs to be updated. Concerning future TC, the following estimates can be provided:
Development costs (nonrecurring):
70 millions Euros per aircraft manufacturer. Cost of production: 85 000 to 220 000 Euros for aeroplane ranging from a typical single aisle aeroplane to a typical twin aisle aeroplane.
Ownership costs: 13 500 to 38 500 Euros per year for aeroplanes ranging from small to large.
La flotta Airbus serie 320 è interessata dal seguente documento: TCDS A.064 ANNEX - Airbus A318, A319, A320, A321 - Special Conditions."
Nelle 31 pagine segnalando come "This document is not exhaustive and it will be updated gradually" ha trattato le questioni:
"Flammable. With respect to a fluid or gas, flammable means susceptible to igniting readily or to exploding. A non-flammable ullage is one where the gas mixture is too lean or too rich to burn and/or is inert per the definition below.
(g) Inerting. A process where a non-combustible gas is introduced into the ullage of a fuel tank to displace sufficient oxygen so that the ullage becomes inert.
(j) Ullage, or Ullage Space. The volume within the tank not occupied by liquid fuel at the time interval under evaluation.
In aggiunta al seguente punto:
Monte Carlo Variables and Data Tables
(a) Fleet average flammability exposure is the percent of the mission time the fuel tank ullage is flammable for a fleet of an aeroplane type operating over the range of actual or expected missions and in a world-wide range of environmental conditions and fuel properties. Variables used to calculate fleet average flammability exposure must include atmosphere, mission length (as defined in paragraph 1.2 Definitions, as FLEET), fuel flash point, thermal characteristics of the fuel tank, overnight temperature drop, and oxygen evolution from the fuel into the ullage. Transport effects are not to be allowed as parameters in the analysis."
