He *may* be right if he is meaning that an airplane will not fly longer at higher alts, Of course we all know that is not true of you are talking about fuel burn. the range is much much longer up high.
If you can fly at Vy (Vx? can't recall) h in level flight you will get the max range. but sice you will be buring less fuel up high...
anyway. I would love to know under what circumstances he is making that claim.
No it does not depend on altitude it depends on speed and endurence
Spend a few minutes in *Aerodynamics for Naval Aviators* (despite the title, it's not just for NA's, but is in fact a very good general aerodynamics and propulsion reference) and it is clear that the answer to the question depends solely on the aircraft's powerplant.
In a nutshell, what you will find is this:
1) a normally aspirated piston powerplant loses propulsive efficiency at a rate that negates the true airspeed advantage of flying higher; therefore the maximum range of an airplane with this type of powerplant will be obtained when it is flown at the lowest practical altitude, and at max L/D AOA.
2) turbo- (or super-) charged recip powerplants, or turbine aircraft, will benefit in increased range from climbing to cruise altitude. The *-charged recip should be flown at or near the critical altitude of the powerplant, and the turbine-powered aircraft higher yet...at the tropopause, for jet aircraft, and somewhat lower for turbopropeller aircraft...depending on the efficiencies of the specific engine-propeller combination.
The Breguet equation is to range as the Carnot equation is to efficiency...a theoretical maximum.
Depends on whether or not it's a jet powered aircraft. A recip will get best range at it's Dmin (L/Dmax) speed, while a jet will get best range at a speed where 75% of the total drag is parasite, and 25% is induced. When the jet aircraft goes higher, induced drag increases, therefore the airspeed at which 75% parasite drag occurs will increase. Due to this, jet aircraft employ very clean configurations. That increased speed is also why range is increased.
I second the recommendation for "Handbook for Naval Aviators."
Don't forget the stratosphere (constant temperature above ~36kft). Engine efficiency is dependent on the temperature increase possible in combustion. The colder the intake air is, the more the temperature can be increase before hitting the thermal limits of materials in the engine.
Above the tropopause, though, there's no more benefit to be had because the temperature no longer decreases with altitude, but the mass of air available to the engine is still decreasing.