Altitude selection

This element is about how high to fly. The answer balances five competing factors: terrain and obstacle clearance, glide distance, VFR cruising altitudes, wind, and oxygen. Pick the altitude that satisfies the regulatory floor, then optimize for the rest.

Five factors compete for the altitude decision: terrain margin, hemispheric rule, wind component, glide picture, and oxygen threshold. Start with the floor and work up.

Hemispheric cruising altitudes

When operating VFR more than 3,000 feet AGL and below 18,000 feet MSL §91.159:

• Magnetic course 0°–179° (eastbound) — odd thousands + 500 (3,500, 5,500, 7,500, 9,500, …)
• Magnetic course 180°–359° (westbound) — even thousands + 500 (4,500, 6,500, 8,500, …)

Important nuances:

• AGL, not MSL — over high terrain (e.g., a 6,000 ft surface), the rule doesn't apply until 9,000+ MSL
• Magnetic course, not heading — a stiff crosswind can mean your heading and course differ by 10°+
• Above 18,000 MSL is Class A — IFR-only
• Below 3,000 AGL the hemispheric rule doesn't apply at all

Minimum safe altitudes and MEF

The regulatory floor lives in §91.119:

• (a) Anywhere — allow for an emergency landing without undue hazard to persons or property on the surface
• (b) Congested area — 1,000 ft above the highest obstacle within 2,000 ft horizontal
• (c) Other than congested — 500 ft above the surface, except over open water or sparsely populated areas (then no closer than 500 ft to any person, vessel, vehicle, or structure)

These are minimums. Cruise at the minimum is a bad plan; build margin.

Recommended margins

• ≥ 1,000 AGL over flat terrain
• ≥ 2,000 AGL over mountainous terrain

Sectional MEF

The Maximum Elevation Figure on each sectional quadrangle is the highest obstacle (terrain + man-made) plus a buffer, in hundreds of feet. Use it to set the cruise-altitude floor for each leg.

Example: planning eastbound across quadrangles with MEFs of 4,200, 5,800, 6,300, 5,100. Highest is 6,300. With 1,000 ft margin: minimum 7,300 MSL. Next legal eastbound altitude is 7,500 per the hemispheric rule. At 5,500, you're below the MEF — illegal AND unsafe.

One gotcha worth carrying: MEF-plus-margin sometimes lands exactly on a hemispheric altitude and sometimes lands just above one — in which case your cruise altitude jumps a full 2,000 ft to the next legal shelf. On a hot-and-high day, that next shelf may be out of climb performance; know before you commit to the leg.

Terrain sets your floor; 14 CFR 91.159 sets which shelves you may cruise on above 3,000 AGL. Drag the course bug and watch the legal ladder flip sides.

Wind aloft and glide

Different altitudes have meaningfully different winds. Check FB (Forecast Winds and Temperatures Aloft) at standard altitudes (3, 6, 9, 12 thousand feet) and pick the altitude with the best wind component for your course. A 10–15 KT difference between altitudes is common; on a 100 NM leg that's 15+ minutes of flight time and several gallons of fuel.

For a typical Cessna 172, the glide ratio is about 9:1 — roughly 9 NM glide per 1,000 ft AGL.

Altitude AGL	Glide distance
3,000	~5 NM
5,000	~9 NM
7,500	~13 NM
10,000	~18 NM

The ideal cruise altitude keeps an emergency airfield within glide throughout the flight.

Trade-off: chasing the best winds can push you into terrain margin, oxygen territory, or against the hemispheric rule. Optimize within the legal envelope, not outside it.

Supplemental oxygen

The altitude triggers §91.211:

Altitude (cabin pressure)	Requirement
> 12,500 MSL for > 30 min	Required crew must use O₂
> 14,000 MSL entire flight	Required crew must use O₂
> 15,000 MSL	Each occupant must be provided O₂

"Provided" means available — passengers don't have to use it, but it has to be there. VFR cruising max is 17,500 (under the 18,000 MSL Class A floor).

Hypoxia is insidious — it degrades judgment and motor control well before any obvious symptoms. Use oxygen earlier than the regulatory minimum if you've been at altitude for a while or notice subtle degradation.

Mountain flying

Mountain-flying altitude decisions overlay the basic rules with unique hazards.

• 2,000 AGL minimum recommended over mountains (vs. 1,000 over flat)
• Cross ridges at 45° — gives 90° escape if downdraft is encountered, not 180°
• Don't fly below ridge level downwind — rotor turbulence and mountain wave develop
• Updrafts upwind, downdrafts downwind — pick the upwind side
• Approach passes from the upwind side
• Lenticular clouds signal mountain wave — strong, smooth-looking, but with rotor below
• Oxygen kicks in above 12,500 MSL sustained; high mountain ranges put you there fast
• Plan extra fuel for higher altitudes (lower TAS at higher DA) and headwind components

Worked examples:

• Eastbound XC over rising terrain. Route MEFs ramp from 4,200 to 6,300 over the last 60 NM. Magnetic course is 095°. Eastbound legal altitudes above the 7,300 ft floor: 7,500 and 9,500. FB winds: 7,500 shows 280 at 18 (tailwind component 16 KT); 9,500 shows 290 at 25 (tailwind 22 KT). Trade-off: 9,500 gives better winds AND more glide range, and the oxygen threshold doesn't trigger (still below 12,500). Choice: 9,500 MSL — best winds, best glide, no O₂ needed, comfortable terrain margin (3,200 ft above the 6,300 MEF).
• Sustained high-altitude crossing. Route requires 14,500 MSL cruise to clear ridge minimums plus the 2,000-AGL mountain margin. At 14,500 MSL > 30 min: required crew (single-pilot) must use supplemental oxygen the entire time. If the airplane isn't equipped — or the bottle is empty — the trip isn't legal as planned. Options: route around at lower altitude, plan a stop for refueling/O₂, or wait for a forecast that allows a lower cruise. Don't try to "stay at 12,499" — the regulatory threshold is exact and cabin altitude tends to drift up with bumps.