“Takeoffs are optional, landings are mandatory” is a well-known aviation adage. “Takeoff is optional, but landing is mandatory” has been cited in print since at least 1986, when it was in the book Landing Gear Design for Light Aircraft by Ladislao Pazmany. The saying was popularized in the book title, Takeoffs Are Optional, Landings Are Mandatory: Airline pilots talk about deregulation, safety, and the future of commercial aviation (1993), by Penelope Grenoble O’Malley.
 
   
OCLC WorldCat record
Landing gear design for light aircraft
Author: Ladislao Pazmany
Publisher: San Diego, Calif. : Pazmany Aircraft Corp., ©1986-
Edition/Format:   Book : English : 1st ed
 
Google Books
Soaring
Volume 50
1986
Pg. 18, col. 2:
“Takeoff is optional, but landing is mandatory.” With this cogent observation Ladislao “Paz” Pazmany launches his third book on airplane design, “Landing Gear Design For Light Aircraft.”
 
31 May 1992, Aiken (SC) Standard, pg. 9C, col. 4:
Finding a suitable spot is important, for as Kuhl explains, “Takeoffs are optional, but landing is mandatory.”
 
OCLC WorldCat record
Takeoffs are optional, landings are mandatory : airline pilots talk about deregulation, safety, and the future of commercial aviation
Author: Penelope Grenoble O’Malley
Publisher: Ames : Iowa State University Press, 1993.
Edition/Format:   Book : State or province government publication : English : 1st ed
 
Google Books
The Hero’s Apprentice:
Essays
By Laurence Gonzales
Fayetteville, AR: University of Arkansas Press
1994
Pg. 201:
Then he comes to the inevitable subject of landing, saying that although take-off is optional, landing is mandatory: ...
 
Global Wit & Worldly Wisdom
99. Fear of Flying (apologies to Erica Jong)
Flight truths:
1. Every takeoff is optional.  Every landing is mandatory.
2. The only time you have too much fuel is when you’re on fire.
3. You start with a bag full of luck and an empty bag of experience.  The trick is to fill the bag of experience before you empty the bag of luck.
4. A “good” landing is one from which you can walk away.  A “great” landing is one after which they can use the plane again.

Taken from CIO (December 1, 2000, p.46) which in turn stole it from Austrailian Aviation.
     
AVweb
Oil Analysis: What It Can and Can’t Do
Oil analysis can help predict some types of problems, but it can’t help with all of them. We clear up some misunderstandings and present some realistic expectations for using it as one of many tools that can keep your engine running well.
June 1, 2003
by Jim Stark
(...)
As the old saw goes, takeoffs are optional, landings are mandatory.
 
Google Books
U.S. Armed Forces Arsenal:
A Guide to Modern Combat Hardware
By Samuel A. Southworth
Cambridge, MA: Da Capo Press
2004
Pg. 154:
As pilots say: “Take off is optional; landing is mandatory.”
   
TribLIVE (Pittsburgh, PA)
Homer Center teacher shares aerial experience in classroom
By Jeff Himler
Published: Friday, September 10, 2004
HOMER CITY—Each fall, when Trish Zimmer returns to her family and consumer science classroom at Homer-Center High School, it’s a new adventure for her students
(...)
For Jack, that episode underscored one of the axioms of air travel: “Take-offs are optional, but landings are mandatory.”
 
26 June 2006, Roswell (NM) Daily Record, “Parade request; a little humor” by John Taylor, pg. A5, cols. 2-3:
Air Force Rules of the Air:
1. Every takeoff is optional. Every landing is mandatory.
2. Flying isn’t dangerous. Crashing is what’s dangerous.
3. The only time you have too much fuel is when you’re on fire.
 
Google Books
Balloon Flying Handbook:
Faa-H-8083-11a
By Federal Aviation Administration
New York, NY: Skyhorse Publishing, Inc.
2012
Pg. ?:
Among the many adages in aviation is “takeoffs are optional, landings are mandatory.” For every flight, there must be a landing, and the balloon pilot should be prepared to execute a landing with skill and precision at any time during the flight.