当雷雨绕飞偏出航路飞行时出现失压，如何确定最低安全高度
与各种最低高度相关的资料：
飞行运行手册 第7章 – 第2节 飞行签派放行 – 10 飞行高度选择规定
10.2 最低安全高度
（1）在任何地方应当保持一个合适的高度，在这个高度上，当航空器动力装置失效应急着陆时，不会对地面人员或财产造成危害。
（2）在人口稠密区、集镇或居住区的上空或者任何露天公众集会上空，航空器的高度不得低于在其600 米（2000 英尺）水平半径范围内的最高障碍物以上300 米（1000 英尺）。
（3）在人口稠密区以外地区的上空，航空器不得低于离地高度150 米（500 英尺）。但是，在开阔水面或人口稀少区的上空不受上述限制，在这些情况下，航空器不得接近任何人员、船舶、车辆或建筑物至150 米（500英尺）以内。
（4）公司运行的任何飞机在昼间按目视飞行规则运行时不得在距地表、山峰、丘陵或其他障碍物300 米（1000 英尺）的高度以下飞行。
（5）公司运行的任何飞机按仪表飞行规则运行时，在距预定航道中心线两侧各25 公里（13.5 海里）水平距离范围内，在平原地区不得距最高障碍物400 米（1300 英尺）的高度以下，在丘陵和山区不得在距最高障碍物600 米（2000 英尺）的高度以下飞行。//MEA
（6）满足下述任一条件前，飞机不得下降到相应的最低航路高度/最低安全高度、最低雷达引导高度或最后进近定位点高度以下：
a．已建立仪表进近程序，在报告的云底高度下进行，且飞行员已获得足够的目视参考，可以将航空器安全引导至机场；
b．飞行在云下实施，可见机场，且在整个机动过程中飞行员可以保持能见机场。
（7）在确定飞机可以使用正常盘旋以正常下降率到接地区之前，飞机不能下降到预计着陆跑道规定的盘旋最低下降高度（MDH）之下。
飞行运行手册 第8章 飞行运作 – 第3节 飞机的安全操作 – 20 严格保持飞行高度
20.4 飞行机组在航线运行中，任何飞行组成员不得下降至任何适用规定的最低高度以下运行飞机，这些高度应包括最低安全高度（MSA）、最低航路高度（MEA）和最低航路外高度（MORA），这些高度可在高中低空航路图上查到；最低下降高度/高（MDA/H），该高度可在机场进近图上查到；最低越障高度（MOCA），该高度可在机场进、离场图上查到；最低引导高度（MVA）；任何局方规定的最低高度。并保证在机上配有两套现行有效的航线资料和QRH 供飞行组查阅。除非：
（1）飞行机组加入了程序进近的初始进近定位点；
（2）飞行机组被雷达引导下降高度；
（3）飞行机组建立了目视参考，由IFR 转化为VFR；
（4）若机长同意，可被雷达引导下降高度至最低引导高度（MVA）。
飞行运行手册 第8章 飞行运作 – 第6节 飞行实施 – 9 巡航
9.1飞行高度规则
（5）各航线最低安全高度的具体限制参见《航线手册》、《航线图》，单发飘降或紧急下降时必须考虑最低安全高度的限制。
JEPPESEN AIRWAY MANUAL – GENERAL – INTRODUCTION – GLOSSARY – DEFINITION
MINIMUM CROSSING ALTITUDE (MCA) — The lowest altitude at certain fixes at which an aircraft must cross when proceeding in the direction of a higher minimum enroute IFR altitude(MEA).
MINIMUM DESCENT ALTITUDE (MDA) (FAA) — Is the lowest altitude specified in an instrument approach procedure, expressed in feet above mean sea level, to which descent is authorized onfinal approach or during circle-to-land maneuvering until the pilot sees the required visual references for the heliport or runway of intended landing.
MINIMUM DESCENT ALTITUDE (MDA) OR MINIMUM DESCENT HEIGHT (MDH) (ICAO) — A specified altitude or height in a non-precision approach or circling approach below which descentmust not be made without the required visual reference.
NOTE 1: Minimum descent altitude (MDA) is referenced to mean sea level and minimum descentheight (MDH) is referenced to the aerodrome elevation or to the threshold elevation if that is morethan 2m (7ft) below the aerodrome elevation. A minimum descent height for a circling approach is referenced to the aerodrome elevation.
NOTE 2: The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path. In the case of a circling approach the required visual reference is the runway environment.
NOTE 3: For convenience when both expressions are used they may be written in the form “minimum descent altitude/height” abbreviated “MDA/H.”
MINIMUM ENROUTE IFR ALTITUDE (MEA) — The lowest published altitude between radio fixes that meets obstacle clearance requirements between those fixes and in many countries assures acceptable navigational signal coverage. The MEA applies to the entire width of the airway, segment, or route between the radio fixes defining the airway, segment, or route.
MINIMUM IFR ALTITUDES (USA) — Minimum altitudes for IFR operations are published on aeronautical charts for airways, routes, and for standard instrument approach procedures. Within the USA, if no applicable minimum altitude is prescribed the following minimum IFR altitudes apply.
a. In designated mountainous areas, 2000ft above the highest obstacle within a horizontal distance of 4NM from the course to be flown; or
b. Other than mountainous areas, 1000ft above the highest obstacle within a horizontal distance of 4NM from the course to be flown; or
c. As otherwise authorized by the Administrator or assigned by ATC.
MINIMUM OBSTRUCTION CLEARANCE ALTITUDE (MOCA) — The lowest published altitude in effect between radio fixes on VOR airways, off airway routes, or route segments which meets obstacle clearance requirements for the entire route segment and in the USA assures acceptable navigational signal coverage only within 22NM of a VOR.
MINIMUM OFF-ROUTE ALTITUDE (MORA) — This is an altitude derived by Jeppesen. The MORA provides known obstruction clearance 10NM either side of the route centerline including a 10NM radius beyond the radio fix reporting or mileage break defining the route segment. For terrain and man-made structure clearance refer to Grid MORA.
GRID MINIMUM OFF-ROUTE ALTITUDE (Grid MORA) — An altitude derived by Jeppesen or provided by State Authorities. The Grid MORA altitude provides terrain and man-made structure clearance within the section outlined by latitude and longitude lines. MORA does not provide for navaid signal coverage or communication coverage.
a. Grid MORA values derived by Jeppesen clear all terrain and man-made structures by 1000ft in areas where the highest elevations are 5000ft MSL or lower. MORA values clear all terrain and man-made structures by 2000ft in areas where the highest elevations are 5001ft MSL or higher. When a Grid MORA is shown as “Unsurveyed” it is due to incomplete or insufficient information. Grid MORA values followed by a +/- denote doubtful accuracy, but are believed
to provide sufficient reference point clearance.
b. Grid MORA (State) altitude supplied by the State Authority provides 2000ft clearance in mountainous areas and 1000ft in non-mountainous areas. MINIMUM RECEPTION ALTITUDE (MRA) — The lowest altitude at which an intersection can be determined.
MINIMUM SAFE/SECTOR ALTITUDE (MSA) (FAA) — Altitude depicted on an instrument chart and identified as the minimum safe altitude which provides 1000ft of obstacle clearance within a 25NM radius from the navigational facility upon which the MSA is predicated. If the radius limit is other than 25NM, it is stated. This altitude is for EMERGENCY USE ONLY and does not necessarily guarantee navaid reception. When the MSA is divided into sectors, with each sector a different altitude, the altitudes in these sectors are referred to as “minimum sector altitudes”.
MINIMUM SECTOR ALTITUDE (MSA) (ICAO) — The lowest altitude which may be used which will provide a minimum clearance of 300m (1000ft) above all objects located in an area contained within a sector of a circle of 46km (25NM) radius centered on a radio aid to navigation.
MINIMUM STABILIZATION DISTANCE (MSD) — The minimum distance to complete a turn manoeuvre and after which a new manoeuvre can be initiated. The minimum stabilization distance is used to compute the minimum distance between waypoints.
MINIMUM VECTORING ALTITUDE (MVA) — The lowest MSL altitude at which an IFR aircraft will be vectored by a radar controller, except as otherwise authorized for radar approaches, departures and missed approaches. The altitude meets IFR obstacle clearance criteria. It may be lower than the published MEA along an airway of J-route segment. It may be utilized for radar vectoring only upon the controller’s determination that an adequate radar return is being received from the aircraft being controlled.
以上是各手册中对各种最低高度的描述和介绍。对于这个问题来说，如果已经不在航路上了，主要参考的就是MORA。
对A330来说，最简单的方法是在FCU上选择CSTR，ND上就会显示GRID-MORA。
另外在航图上也能找到。但注意国内高空图上只有网格MORA，在JEPPESEN航图上有MORA和GRID-MORA两种，根据距离航路的距离是否大于10NM来决定使用哪一个。
最后实在找不到还可以向ATC询问。
